* Bump API patch number and header version number to 20 for this
update.
Github Issues:
* Replaced existing reference pages by text automatically extracted
from the specification source, or generated from vk.xml in some
cases. This isn't a complete solution for the reference pages, but
puts them in a much better state. The ref pages (only) are now
placed under a CC BY open source license, which is more current than
the obsolete license previously used.
Further improvements to the pages should not edit them directly, but
instead concentrate on the specification source from which the ref
pages are being extracted (public issues 44, 55, 160; internal issue
389).
* Bump API patch number and header version number to 18 for this
update.
Github Issues:
* Added "queue operation" terminology, and modified spec to actually
use this terminology (public issue 155). The act of submitting a
piece of work to a queue now generates "operations" for the queue to
execute, including operations to wait on/signal semaphores and
fences. Synchronization waits on these operations, making execution
dependency chains more obvious for semaphores and fences (though
additional work is still needed here). These changes include:
** Overview of "queue submission" commands in chapter
<<devsandqueues-submission>>.
** Updated descriptions for fence and semaphore waits and signals in
the synchronization chapter <<synchronization-semaphores-waiting>>,
<<synchronization-semaphores-signaling>> and
<<synchronization-fences-waiting>>.
** Clarifications to semaphore and fence operation within queue
submission functions.
** New glossary terms.
** Moved device idle and queue wait idle to synchronization chapter in
order to describe them in terms of other synchronization
primitives.
** Clarifications to semaphore and fence operation allowed removal of
the "implicit ordering guarantees" section, as this information is
now wholly covered where these primitives are described.
*** The "host writes" section of this is still there for now - in its
own section. This could probably be merged into other sections
later.
*** Modified fundamentals chapter on queue ordering to make sense in
context of the new changes, and avoid duplication.
<<fundamentals-queueoperation>>
* Added "aspect" and "component" definitions to the glossary, and made
sure these terms are referenced correctly (public issue 163).
* Update valid usage for ftext:vkGet*ProcAddr to only include
conditions that must be met to get a valid result. In particular,
it's okay to call flink:vkGetDeviceProcAddr with any string and will
get a code:NULL if that string is not a core Vulkan function or an
enabled extension function (addresses but does not fully close
public issue 214).
* Change the WSI extension dependencies to refer to version 1.0 of the
Vulkan API, instead of the pre-1.0-release internal revisions
numbers (public issue 238).
* Specified that <<interfaces-fragmentoutput,undeclared fragment
shader outputs>> result in undefined values input to the blending
unit or color attachment (public issue 240).
Internal Issues:
* Better documented that the registry XML "optional" tag for values
only applies when that value is the size of an array (internal issue
335).
* Add a stronger definition for the valid usages of
VkSpecializationMapEntry.size in the
<<pipelines-specialization-constants,Specialization Constants>>
section (internal issue 345).
* Change code:OpName to code:OpDecorate (along with appropriate
syntax) for vertex shader built-ins (internal issue 368).
* Add missing ref pages (those which are not currently stubs) to
apispec.txt for the single-page version of the ref pages (internal
issue 378).
Other Commits:
* Fix example in the <<descriptorsets,Descriptor Sets>> section to use
M, N, and I, describing set, binding, and index, consistently
throughout the example code.
* Bump API patch number and header version number to 13 for this
update.
Github Issues:
* Improve the description of ename:VK_PRESENT_MODE_FIFO_RELAXED_KHR in the
VK_KHR_surface extension (public issue 174).
* Clarify use of etext:*_SIMULTANEOUS_USE_BIT for secondary command
buffers (public issue 182).
* Fix typos in VK_KHR_wayland_surface extension where code:wl_device was
used instead of code:wl_display (public issue 193).
* Replaced {apiname} with ``Vulkan'' in XML validity statements (public
issue 199).
* Fix dead links for WSI handle types (public issue 200).
*** N.b. this needs to be done in WSI branches as well ***
* Use "signaled" instead of "signalled" spelling everywhere (public issue
201).
*** N.b. this needs to be done in WSI branches as well ***
* Move readme.pdf target directory for XML schema documentation into the
target generation directory, instead of leaving it checked into the spec
source tree (public issue 203).
*** N.b. need to add generated PDF to registry index/1.0 page
* Fix duplicate 'which which' typo in description of
elink:VkCommandPoolResetFlagBits (public issue 204).
* Move the <<Programmable Primitive Shading>> section up one level, out of
the <<drawing-primitive-topologies,Primitive Topologies>> section
(public issue 209).
Internal Issues:
* Clarify in the <<pipelines-cache,Pipeline Cache>> section that
implementations should not manage the size of pipeline cache (internal
issue 192).
* Deprecate the concept of device layers and associated commands (internal
issue 255).
* Remove ename:VK_INCOMPLETE from the list of possible result codes of
flink:vkGetPhysicalDeviceSurfaceCapabilitiesKHR (internal issue 314).
* Add missing std140/std430 rule: the base alignment of a member following
a structure is a multiple of the structure's base alignment (internal
issue 321).
* Fixes naming of the single elink:VkColorSpaceKHR enum from
ename:VK_COLORSPACE_SRGB_NONLINEAR_KHR to
ename:VK_COLOR_SPACE_SRGB_NONLINEAR_KHR in XML/header and the
VK_KHR_swapchain and VK_KHR_surface extensions to match the style of the
typename (space and color are two words, not one) (internal issue 322).
* Make it clear that code:LocalInvocationID should only be applied to an
input variable and normalize the language describing
code:LocalInvocationID to the language for other compute shader
variables in the <<interfaces-builtin-variables,Built-in Variables>>
section, and add normative language (internal issue 323).
* Clarify in the <<fundamentals-returncodes,Return Codes>> section that
the result pointer may be modified for specific commands, even if a
runtime error is returned (internal issue 324).
* Bump API patch number and header version number to 12 for this
update.
Github Issues:
* Change valid usage statements intended to be "sub-points" to
be actual sub-points (public issue 66).
* Replace double negation in description of
slink:VkRenderPassBeginInfo::pname:pClearValues (based on public
merge 142).
* Cleanup minor typos in spec, ref pages and XML, including those
proposed in public pull requests 144, 150, 151, 167, 168, 181, and
186.
* Use *strict subset* in describing the partial order of memory
property types for slink:VkMemoryType, and update the style guide
accordingly (public issue 190).
* Fix various "a image" -> "an image" typos (public issue 191).
* Note in the <<fundamentals-validusage,Valid Usage>> and
<<extensions-interactions,Extension Interactions>> sections that
structures defined by extensions which may be passed in structure
chains using the ptext:pNext member must: include initial
ptext:sType and ptext:pNext members (public issue 192).
Internal Issues:
* Remove duplicate language from the description of the pname:fence
parameter to flink:vkQueueSubmit and improve validity language
(internal issue 91).
* Added documentation for "optional" attribute to XML readme.tex/pdf
(internal issue 149).
* Clarify the host-side data validity rules and behavior of
flink:vkFlushMappedMemoryRanges and
flink:vkInvalidateMappedMemoryRanges (internal issue 266).
Other Commits:
* Added clarification to flink:vkCmdFillBuffer regarding the use of
ename:VK_WHOLE_SIZE.
* Fixed and documented implementation of "validextensionstructs"
attribute. in XML processing scripts and readme.tex/pdf.
* Add missing validity statements to flink:vkResetEvent and
flink:vkCmdResetEvent.
* Fix validity for the
ename:VK_FORMAT_FEATURE_SAMPLED_IMAGE_FILTER_LINEAR_BIT flag.
Correct all the draw/dispatch commands to mention optimally tiled
images as well as linear tiled images, and say image VIEWS instead
of images. Add validity statement to flink:vkCmdBlitImage
* Replace the {apiname} macro with hardcoded "Vulkan", now that we've
committed to that name.
* Add the VK_AMD_rasterization_order extension to vk.xml.
* Bump API patch number and header version number to 11 for this
update.
Github Issues:
* Clarify the WSI extension language by switching from the fuzzier
"ownership" language to more-consistent "acquire" language (public
issue 117).
* Clarify that memory barriers apply to all commands in the dependency
chains in the flink:vkGetRenderAreaGranularity command and the
<<synchronization-execution-and-memory-dependencies,Execution And
Memory Dependencies>> section (public issue 132).
* Clarify that a queue family is a set of queues in the
<<fundamentals-execmodel,Execution Model>> section (public issue
166).
* Removed requirement from valid usage language that
VkPresentInfoKHR::waitSemaphoreCount must be greater than 0 (public
issue 171).
* Fix broken internal links, describe structures consistently, use
consistent style for SPIR-V codewords, and tag normative terms that
were missing asciidoc tags (public issue 183 and ancillary
markup/normative language fixes).
* Fix typos for slink:VkPhysicalDeviceLimits member names in
slink:VkImageCreateInfo validity language (public issue 184).
Internal Issues:
* Document that the requested patch version number specified as part
of slink:VkApplicationInfo::pname:apiVersion is ignored when
creating an instance (internal issue 176).
* Clarify handling of extension structs in the
<<fundamentals-validusageValid Usage>> section (internal issue 254).
* Update required slink:VkImageFormatProperties::pname:maxMipLevels to
be limited to the maximum allowed mipmap pyramid size corresponding
to the actual maximum supported size for the format (internal issue
256).
* Modify the <<features-extentperimagetype,Allowed Extent Values Based
On Image Type>> section so the allowed maximum extent is the maximum
image dimension supported for each dimension of the type of texture
being queried (internal issue 257).
* Clarify in the <<spirvenv-module-validation,Validation Rules within
a Module>> section that at least one of the code:LocalSize execution
mode or code:WorkgroupSize decoration is required for each compute
shader entry point in a shader module (internal issue 279).
* Add validity rules for formats in flink:vkCmdClearColorImage and
flink:vkCmdClearDepthStencilImage (internal issue 283).
* Clarify that slink:VkImageFormatProperties::pname:maxResourceSize is
an upper bound, and that it may not be possible to create an image
anywhere near that size (internal issue 284).
Other Commits:
* Fix various minor markup errors reported by validation scripts.
* Change copyright from Khronos Free Use License to Apache 2.0 license
on relevant script/XML/header files. This does not affect the
specification source copyright.
* Bump API patch number and header version number to 7 for this
update.
Github Issues:
* Fix slink:VkSpecializationMapEntry example to avoid C/C++ strict
aliasing issues (public issue 14).
* Clarify the meaning of "matching" in flink:vkCmdBindDescriptorSets
validity language (public issue 33).
* Add stub reference pages so xrefs to not-yet-written pages don't
generate 404 errors. However, the actual content of these pages
still needs to be filled in as time allows (public issue 44, but
does not close that issue out).
* Remove incorrect validity statement for
flink:vkGetImageSparseMemoryRequirements (public issue 85).
* Reword the
<<features-limits-bufferImageGranularity,bufferImageGranularity>>
feature in terms of "aliasing", and clarify that it applies to
bindings in the same memory object (public issue 90).
* Clarify the relationship of the slink:VkPhysicalDeviceLimits
pname:maxViewportDimensions and pname:viewportBoundsRange limits
(public issue 92).
* Specify sparse unbound texture replacement in the
<<textures-texel-replacement,Texel Replacement>> section
independently of robust buffer access language (public issue 100).
* Add the <<fundamentals-architecture-model,Architecture Model>>
section to explain architecture constraints Vulkan has chosen to
accept in order to enable portable and performant code (public issue
122).
* State that an object must not be destroyed until *all* (not *any*)
uses of that object have completed (public issue 123).
* Minor editorial cleanup (public issues 129, 134, 146, 148).
* Add validity language for layer and extension names to
slink:VkDeviceCreateInfo matching that used for
slink:VkInstanceCreateInfo (public issue 130).
* Clean up terminology for the case when the bits set in one bitmask
are a subset of the bits set in another bitmask (public issue 138).
* Document that input attachments are UniformConstant not Input, in
the <<interfaces-inputattachment,Fragment Input Attachment
Interface>> section (public glslang bug 169).
Internal Issues:
* Add max enum values to "flag bits" enums (internal issue #136).
* Clarify language around the various uses of the term "block" in the
<<appendix-compressedtex-bc,Block Compressed Image Formats>> section
(internal issue #202).
* Removed "expand" dependency from <enums> groups in vk.xml and added
auto-generation code in the scripts to infer it instead, to ensure
consistency. This caused renaming of sname:VkColorSpaceKHR and
sname:VkPresentModeKHR etext:BEGIN_RANGE (etc.) tokens, but those
tokens are metadata, not part of the API, and the Vulkan WG is OK
with this change. This change adds ranges to two additional enums
that were missing them due to not defining the "expand" attribute
(internal issue 217).
* Tweak makefile to generate ref page nroff (.3) files in the right
output directory, working around an a2x limitation (internal issue
223).
Other Commits:
* Add validity requirements for flink:vkCmdCopyQueryPoolResults
pname:dstBuffer parameter.
* Fix ref page build to generate .3 targets in the right output
directory.
* Bump API patch number and header version number to 6 for this
update.
Github Issues:
* Define 'invocation group' for compute and graphics shaders. Cleanup
definition and use of 'workgroup', and add glossary entries (public
issue 1).
* Various minor editorial fixes (public issue 33).
* Clarify locations for block members in the
<<interfaces-iointerfaces-locations,Location Assignment>>
section (public issue 45).
* Editorial fixes for <<commandbuffer-allocation,Command Buffer
Allocation>> section (public issues 54, 59).
* Clarify behavior of depth test in the <<fragops-depth,Depth
Test>> section (public issues 80, 81).
* Remove discussion of return codes from
flink:vkGetPhysicalDeviceSparseImageFormatProperties and
flink:vkGetImageSparseMemoryRequirements, which don't return values
(public issue 82).
* Allow flink:vkCmdDrawIndirect and flink:vkCmdDrawIndexedIndirect
pname:drawCount of 0, as well as 1, when the multiDrawIndirect
feature is not supported (public issue 88).
* Remove confusing wording in the <<features-limits,Limits>>
section describing the slink:VkPhysicalDeviceLimits
pname:minTexelBufferOffsetAlignment,
pname:minUniformBufferOffsetAlignment, and
pname:minStorageBufferOffsetAlignment members as both minimums and
maximums (public issue 91).
* Clarified that only the RGB components should be affected in places
where sRGB is referred to in the spec, such as ASTC formats. Minor
re-wording to avoid "color space" when actively incorrect, now that
we refer to the Data Format Spec which actually makes a distinction
between color space and transfer function (public issue 94).
* Treat pname:pPropertyCount == 0 consistently in
flink:vkEnumerateInstanceLayerProperties and
flink:vkEnumerateDeviceLayerProperties (public issue 99)
* Cleanup minor editorial issues in chapters 14-17 (public issue 100).
* Clarify definition of flink:vkEnumerateInstanceExtensionProperties
and flink:vkEnumerateDeviceExtensionProperties (public issue 101).
* Define the flink:vkEnumerateInstanceExtensionProperties and
flink:vkEnumerateDeviceExtensionProperties pname:pLayerName
parameter to be a pointer to a null-terminated UTF-8 string (public
issue 101).
* Rearrange "Missing information" references in mandatory format
tables (public issue 101).
* Clarify that the enumerated extensions returned by
flink:vkEnumerateInstanceExtensionProperties and
flink:vkEnumerateDeviceExtensionProperties will only include
extensions provided by the platform or extensions implemented in
implicitly enabled layers (public issue 101).
* Miscellaneous editorial fixes. Include the Vulkan spec patch number
in the PDF title. Fix label on <<fig-non-strict-lines,Non
strict lines>> diagram. Use more easily distinguished symbols in
tables in the <<features-required-format-support,Required
Format Support>> section. Don't require FQDNs used as layer names be
encoded in lower case if not possible, in the
<<extensions-naming-conventions, Extension and Layer Naming
Conventions>> section (public issues 101, 119, 121).
Internal Issues:
* Fixed excessive spacing in tables in XHTML (internal issue 18).
* Clarify that ename:VK_COMMAND_BUFFER_USAGE_ONE_TIME_SUBMIT_BIT
applies to secondary command buffers. Previously spec only referred
to the members of pname:pCommandBuffers being affected by this bit.
Added a separate slink:VkSubmitInfo Valid Usage restriction
specifying that ename:VK_COMMAND_BUFFER_USAGE_ONE_TIME_SUBMIT_BIT
also applies to any secondary command buffers that are recorded into
the primary command buffers in pname:pCommandBuffers (internal issue
106).
* Clarify that slink:VkDeviceCreateInfo::pname:pEnabledFeatures can be
NULL (internal issue 117).
* Remove "the value of" where it is redundant (e.g. speaking of an API
parameter, struct member, or SPIR-V variable, but not when speaking
of color components) (internal issue 175).
* Forced patch version to always be 0 in the header. Add a
"VK_API_VERSION_<major>_<minor>" macro for people to use to do the
right thing. Add a VK_HEADER_VERSION which captures the header
release number independent of the spec patch number (internal issue
176).
* Correct description of
slink:VkPipelineShaderStageCreateInfo::pname:pName to "a pointer to
a null-terminated UTF-8 string" (internal issue #197).
Other Commits:
* Updated DataFormat spec reference to the new date for revision 5 of
that spec.
* Fixed KEEP option (to retain LaTeX intermediate files) in the
Makefile to be included when edited there, as well as set on the
command line.
* Reserve and add "VK_IMG_filter_cubic" to the registry, and implement
script functionality to add and remove validity from existing
functions. Includes schema and readme changes.
* Update GL_KHR_vulkan_glsl so push_constants do not have descriptor
sets.
* Bump API patch number to 5 for this update.
Github Issues:
* Correctly describe slink:VkPhysicalDeviceProperties pname:deviceName
member as a string, not a pointer to a string. Also one typo fix for
"hetereogeneous" (public issue 4).
* Replace maynot: macro with may: not, and "may: or maynot:" with
"may:" (public issue 4).
* Clarify that redundantly setting the state of a fence or event has
no effect (public issue 4).
* Minor fixes to ref pages to track descriptions of memory bits that
changed in the core spec. Fix name of a member in the description of
sname:sname:VkPipelineMultisampleStateCreateInfo (public issues 8,
13).
* Remove redundant validity statement for
sname:VkGraphicsPipelineCreateInfo::pname:stageCount (public issue
14).
* Fix typos in chapters 7-9 (public issue 14).
* Clarify the example demonstrating the behavior of
code:OpMemoryBarrier in the
<<shaders-execution-memory-ordering,shader memory acces
ordering>> section (public issue 16).
* Specify that freeing mapped memory implicitly unmaps the memory in
the description of flink:vkFreeMemory (public issue 17).
* Forbid allocation callbacks from calling into the API in the
<<memory-allocation,memory allocation>> section (public issue
20).
* Add missing validity rules about size being greater than 0 and
offset being less than size of object. Fix
flink:VkMappedMemoryRange's misinterpretation of offset (public
issues 27, 31).
* Add validity rule disallowing overlapping source/destination
descriptors in flink:VkCopyDescriptorSet (public issue 32).
* Clarify that array and matrix stride has to be a multiple of the
base alignment of the array or matrix in the
<<interfaces-resources-layout,Offset and Stride Assignment>>
section (public issue 38).
* Correct parenthesis floor nesting error in equation for
<<textures-RGB-sexp,RGB to shared exponent conversion>>.
Clarify case of when exp' is forced to 0, avoiding log2(0) undefined
problem (public issue 40).
* Remove redundant statement from the code:FragDepth description in
the <<interfaces-builtin-variables,Built-In Variables>>
section (public issue 47).
* Define the clamping of the
<<textures-level-of-detail-operation,bias added to the scale
factor>> by linking to the slink:VkPhysicalDevice feature
pname:maxSamplerLodBias (public issue 64).
* Fix typo "optimal linear resources" and clarify the set of resources
<<features-limits-bufferImageGranularity,the
pname:bufferImageGranularity resource>> applies to (public issue
67).
* Replace 'descriptor accessed by a pipeline' language for
sname:VkDescriptorSetAllocateInfo with more precise phrasing about
binding a descriptor set before a command that invokes work using
that set (public issue 69).
* tstripadj.svg contained an Inkscape tag which caused Firefox and IE
11 to fail to render it, and was illegal SVG. Generating Plain SVG
from the Inkscape SVG source fixes this (public issue 70).
* Fix validity for sname:VkVertexInputBindingDescription and
sname:VkVertexInputAttributeDescription numbers (public issue 72).
Internal Issues:
* Clarify the meaning of
ename:VK_FORMAT_FEATURE_SAMPLED_IMAGE_FILTER_LINEAR_BIT in
elink:VkFormatFeatureFlagBits with respect to depth compare
(internal issue 107).
* Added a note explaining that ename:VK_QUEUE_TRANSFER_BIT may or may
not be reported for a queue family that already supports
ename:VK_QUEUE_GRAPHICS_BIT or ename:VK_QUEUE_COMPUTE_BIT as the
former is a strict subset of the latter ones (internal issue 116).
* Add validity language for sname:VkDescriptorSetAllocateInfo about
exceeding the descriptor pool capacity (internal issue 140).
* Add ename:VK_INCOMPLETE success code for
flink:vkEnumeratePhysicalDevices query (internal issue 163).
Other Commits:
* Add the VK_NV_glsl_shader extension definitions to the API.
* Update GL_KHR_vulkan_glsl with 1) origin_upper_left as default 2)
specialization array constant semantics.
* Corrected/updated Data Format Specification date.
This is for people to review edits for @philiptaylor's points, but I'll
probably pull equivalent changes back into the private repo and merge
there before bringing it back to github in one of the (hopefully) weekly
updates.
[ChangeLog] This bundles up low-hanging editorial issues from public issue
101. Some items from that issue are more complex and remain to be done.
There's also some related cleanup of the extensions appendix to clarify the
distinction between a Vulkan API command, and the functions and function
pointers corresponding to that command which may be provided by the API
loader and runtime.
Remaining todo from #101 :
- [ ] 24.5.1 "Ab, Yb, Zb" -> "Xb, Yb, Zb".
- [ ] 29.2 "explicitly enabled layers" clarification.
- [ ] 30.3.3 Confusingly similar tiny circle symbols in format tables.
- [ ] 30.3.3 Missing information about some formats.
- [ ] C.3 Clarify or punt on "lower-case UTF-8" in FQDN layer names.
* Bump API patch number from 3 to 4 for the first public update to the
spec. Add patch number to the spec title (this will be done
automatically from XML, later).
* Fixes for numerous editorial issues. Regularize descriptions of
variable-length array queries. Properly tag enumerants so they come
out in the right font (many were mislabeled in usage tags in vk.xml,
or not tagged). Spelling and markup corrections (public issue 4).
* Fix typos and clearly separate description of different types of
memory areas (public issue 5).
* Use standards-compliant preprocessor guard symbols on headers
(public issue 7).
* Note that Github users can't currently set labels on issues, and
recommend a fallback approach (public issue 15).
* Use latexmath prefix on len= attributes (public issue 29).
* Make flink:vkCmdUpdateBuffer pname:dataSize limit consistent (public
issue 65).
* Add VK_KHR_mirror_clamp_to_edge extension to core API branch, as an
optional feature not introducing new commands or enums (internal
issue 104).
* Cleanup invariance language inherited from the GL specification to
not refer to nonexistent (GL-specific) state (internal issue 111).
* Modify the flink:vkCmdDrawIndexed pname:vertexOffset definition to
not be the "base offset within the index buffer" but rather the
"value added to the vertex index before indexing into the vertex
buffer" (internal issue 118).
* Fix drawing chapter in the "Programmable Primitive Shading" section
where it described categories of drawing commands. It referenced
flink:vkCmdDrawIndexed twice. Replace the second reference with
flink:vkCmdDrawIndexedIndirect (internal issue 119).
* Typo fixed in <<sparsememory-examples-advanced,Advanced Sparse
Resources>> sparse memory example (internal issue 122).
* Add flink:VkDisplayPlaneAlphaFlagsKHR to <require> section of
VK_KHR_display extension (internal issue 125)
* Add missing optional="false,true" to
flink:vkGetImageSparseMemoryRequirements
pname:pSparseMemoryRequirementCount parameter (internal issue 132)
* Rename ename:VK_STRUCTURE_TYPE_DEBUG_REPORT_CREATE_INFO_EXT to
ename:VK_STRUCTURE_TYPE_DEBUG_REPORT_CALLBACK_CREATE_INFO_EXT
(internal issue 133)
* Fix a handful of broken cross-references in the
<<samplers,Samplers>> chapter (internal issue 134).
* Fix "Input Attachement" GLSL example to use correct syntax (internal
issue 135).
* Update XML schema and documentation to accomodate recently added
attributes for validity. Add some introductory material describing
design choices and pointing to the public repository to file issues.
* Put include of validity in the core spec extensions chapter on its
own line, so that asciidoc is happy.
* Fix vertexOffset language to specify that it's the value added to
the vertex index before indexing into the vertex buffer, not the
base offset within the index buffer.
* Fix error in the description of flink:vkCmdNextSubpass.
Jon Leech