If an update to an image subresource is entirely superseded by a
following update, the former update is dropped.  This change adds a perf
warning for this.

Bug: angleproject:3461
Change-Id: Iaf4984ba52f3dfc86d5cc87f8be82123badebd04
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2613203Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>