Done after noticing a bug in the Vulkan backend where masked (and
possibly scissored) color clears were only clearing the first
attachment.

Bug: angleproject:2361
Change-Id: I471d337b1c3a3a5e17748690ae2222a6351773ac
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1544971Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>