* Bump API patch number and header version number to 27 for this update.
Github Issues:
* Weaken flink:vkGetPipelineCacheData invariance conditions; previous
conditions were stronger than agreed and can't be guaranteed (public
issue 280).
* Add link to "Vulkan Loader Specification and Architecture Overview"
document to Normative References section (public issue 359).
Internal Issues:
* Be more clear in the <<interfaces-resources-layout-std140, uniform
buffer layout>> section that block offsets can be out of order
(internal issue 396).
* Document that extension authors should add support for their extensions
to the validation layers (internal issue 398).
* Clarify that the valid range of depth clear values should be limited
to the 0..1 range and that copies to depth aspect must also be in this
range (internal issue 412).
* Specify ``a'' vs. ``an'' use in the style guide (internal issue 432).
* Increase the maximum pname:nonCoherentAtomSize value in the
<<features-limits-required,Required Limits>> section from 128 to 256
(internal issue 435).
* Fix vk_platform.h for compiler errors on some Android platforms
(internal issue 441).
* Clarify that slink:VkPhysicalDeviceFeatures::pname:pEnabledFeatures ==
`NULL` disables all features, including the "required" feature
pname:robustBufferAccess (internal issue 479).
Other Issues:
* Expand style guide and make it more self-consistent.
* Use ISO 8601 date format everywhere.
* Emphasise the correct way of using
slink:VkSurfaceCapabilitiesKHR::pname:maxImageCount.
* Added +VK_EXT_validation_flags+ extension for validation flag mechanism.
* Fix an <<credits,author credit>> to include their current employer.
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