We noticed a significant hotspot in vkAllocateDesctiptorSets. The app
was repeatedly cycling through a few combinations of active textures.
For each state change in ANGLE we were allocating a new desctiptor set.
This in turn would trigger internal driver memory allocation and cause
jank. Using a cache avoids allocations entirely since the application
is rotating through a stable set of textures.

The descriptor cache is stored in each program. It is indexed by a set
of 32-bit serials. Each texture generates a unique serial for every
combination of VkImage and VkSampler that the texture owns. The texture
descriptor is refreshed every time a texture changes or is rebound.

The descriptor cache is accessed via an unoredered map with the texture
serial sets as the hash key. We also store the maximum active texture
index in the cache key so we don't need to hash and memcmp on all 64
active textures.

This will currently fail if more than MAX_UINT serials are generated.
But that number is high enough that it shouldn't be possible to hit
in practice in a practical amount of time.

Requires shifting the texture sync to ContextVk so we can get the new
serial after the textures are updated. And to make sure to update the
image layouts even if the descriptors are not dirty.

Improves performance of the T-Rex demo. Also improves the score of the
texture state change microbenchmark by about 40%.

Bug: angleproject:3117
Change-Id: Ieb9bec1e8c1a7619814afab767a1980b959a8241
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1642226Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>