The Vulkan backend no-ops such clears before flushing deferred clears.
Deferred clears are actually not gathered when doing a scissored clear,
so this is not an issue.  An ASSERT is added along with a regression
test.

Bug: angleproject:4836
Change-Id: I5ea5bab499ced41e13023ffb6b821e3caefb9ab2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2453466
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>