These bits will be used to aid in invalidation of
multisampled-render-to-texture attachments.

Bug: angleproject:4836
Change-Id: Ib2b438386f8cd8c057bc0ef16144b9d2ddbc1594
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2444097
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>