Especially with emulated formats and robust resource init, a clear is
staged that's often superseded by a data upload to the same subresource.
This change ensures that superseded updates are dropped to avoid
unnecessary GPU work.

Bug: angleproject:4691
Change-Id: I697ccd438b92fd2fff17a5800550694658c95c54
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2262574
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>