To support YUV conversion we need to track the use of immutable samplers
in the descriptor set layout.

Bug: b/155487768
Change-Id: Ic7dc6a08551f5125c4a519b5cfada312f95ab914
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2225423
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Ian Elliott <ianelliott@google.com>