This is a foundational refactor in preparation for implementing sampler
objects in the Vulkan backend.

Bug: angleproject:3208
Change-Id: I5970f141d3f825aee1f8b713be8e162c7d0f8bbe
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1710961Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Tobin Ehlis <tobine@google.com>