Add immutable samplers to descriptor set layout 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>