This is the initial implementation to get GLES3.0 Sampler Objects
working on the Vulkan backend.
When samplers are dirty, a corresponding Vulkan sampler object will be
created with associated state and textures are flagged as dirty anytime
sampler bindings change.
Then when handling textures dirty, any bound sampler objects are pulled
from active texture units. When sampler objects are bound, their state
is used instead of the built-in texture's sampler state.

This change also adds a test that uses the same texture object with
different sampler objects to test a corner case that dEQP misses.

Bug: angleproject:3208
Change-Id: I643d9c9d5cb92fecc02dad815a07bcf349534c70
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1706897
Commit-Queue: Tobin Ehlis <tobine@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>