This uses a packed sampler description to re-use samplers for multliple
VkImages. The samplers will persist for the lifetime of RendererVk. In
the future we could look at doing cache eviction for large object
counts.

Reduces the active VkSampler cache in Manhattan from over 1200 to 9.
Also should reduce the number of VkSamplers used with Chrome.

Bug: angleproject:4491
Change-Id: Idca00e4ed8cb660a0865281544aaa57cf884bbdb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2160771Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Tobin Ehlis <tobine@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>