* Bump API patch number and header version number to 57 for this update.
Github Issues:
* Fix error in description of ename:VK_FORMAT_ETC2_R8G8B8A8_SRGB_BLOCK
block size (public issue 342).
* Update documentation of ename:VK_PIPELINE_CREATE_ALLOW_DERIVATIVES_BIT
to explicitly mention both graphics and compute pipelines (public issue
525).
Internal Issues:
* Document that
slink:VkPhysicalDeviceLimits::pname:framebufferColorSampleCounts does
not cover integer formats (internal issue 550).
* Add a note under slink:VkImageViewCreateInfo describing how values meant
for one format can be sanitized when used via another format (internal
issue 927).
* Add valid usage statements to ftext:vkCmd* documenting that image
subresources used as attachments must not be accessed as non-attachments
in a render pass (internal issue 929).
* Remove obsolete 'validextensionstructs' attribute from +vk.xml+, the XML
schema, and the schema documentation (internal issue 946).
New Extensions:
* `VK_AMD_mixed_attachment_samples`
* `VK_EXT_post_depth_coverage`
* `VK_KHR_relaxed_block_layout`
* `VK_NV_depth_range_unrestricted`
Jon Leech