* Bump API patch number and header version number to 50 for this update.
Github Issues:
* Fix numerous minor issues with the VK_EXT_debug_report extension (public
issues 478, 483, 486, 489, 490).
Internal Issues:
* Update flink:vkAllocateDescriptorSets to specify conditions under which
to return ename:VK_ERROR_FRAGMENTED_POOL or
ename:VK_ERROR_OUT_OF_POOL_MEMORY instead of
out-of-host/out-of-device-memory, and improve the
<<fundamentals-errorcodes, description of those errors (internal issue
654).
* Add a NOTE documenting that flink:vkAcquireNextImageKHR can only signal
a single semaphore, and how to deal with that when multiple physical
devices in a logical device need to wait on it (internal issue 730).
* Improve description of pname:pNext chains of
slink:VkPhysicalDeviceImageFormatInfo2KHR and
slink:VkImageFormatProperties2KHR (internal issue 814).
* Clean up math markup issues in the <<textures, Image Operations>>
chapter (internal issue 818).
* Update validusage target to use more robust code for preprocessing, by
making direct use of Asciidoctor's preprocessor. Added uniqueItems check
to JSON vu schema and add clean_validusage target (internal issue 826).
* Update style guide to prohibit writing non-self-contained (on a single
bullet point) Valid Usage statements, and modify offending Valid Usage
statements in the Specification to match, to assist with automatic
extraction for the validation layers (internal issue 828).
* Add ename:VK_VALIDATION_CHECK_SHADERS_EXT to elink:VkValidationCheckEXT
of the `VK_EXT_validation_flags` extension, to selectively disable
shader validation.
* Remove duplicate valid usage statement for slink:VkImageMemoryBarrier.
* Modify reflow.py script to place VUID tag anchors standalone on a line
following their corresponding bullet point, and reflow the spec text
accordingly (this had been pending since the initial tag deployment).
New Extensions:
* `VK_AMD_texture_gather_bias_lod`
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