UV plane's coord should be half of that of Y plane for NV12.
Bug: chromium:1415832
Change-Id: Icee851e61eca1f7da291285100f8edcce482b511
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/119860
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Jie A Chen <jie.a.chen@intel.com>
Commit-Queue: Jie A Chen <jie.a.chen@intel.com>
Reviewed-by: Ben Clayton <bclayton@google.com>