* Update release number to 91.
Public Issues:
* Update Ubuntu subsystem build instructions in `BUILD.adoc` (public pull
request 624).
* Delete the `VK_KHR_mir_surface` extension from the Specification and
XML, due to EOL of the only driver known to have supported it, and
near-EOL of Mir itself (public issue 814).
* Fix options for some figures that were using old ones (public pull
request 841).
* Fix various accidentally repeated words (public pull request 843).
* Use `time.process_time()`, introduced in Python 3.3, in the scripts
instead of `time.clock()`, which will be removed in Python 3.8 (public
pull request 844).
Internal Issues:
* Update valid usage statements for
`VK_ANDROID_external_memory_android_hardware_buffer` in
slink:VkMemoryAllocateInfo,
slink:VkImportAndroidHardwareBufferInfoANDROID, and
flink:vkGetAndroidHardwareBufferPropertiesANDROID to actually be
verifiable (internal issue 1419).
* Update valid usage statements for
`VK_ANDROID_external_memory_android_hardware_buffer` in
slink:VkMemoryAllocateInfo, slink:VkImageCreateInfo, and
slink:VkImageViewCreateInfo to move valid usage statements in
doubly-nested bullet points up one level, accomodating limitations of
the valid usage extraction script that creates `validusage.json`
(internal issue 1434).
* Fix typo etext:VK_ACCESS_SHADING_RATE_IMAGE_BIT_NV to the correct
ename:VK_ACCESS_SHADING_RATE_IMAGE_READ_BIT_NV.
* Add missing etext:VK_STRUCTURE_TYPE_* tokens to appendices for
extensions missing them.
New Extensions:
* `VK_AMD_memory_overallocation_behavior`
* `VK_NV_ray_tracing`, replacing `VK_NVX_raytracing`
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