Vulkan: Add test and fix clear on RGB8 with color masks Fixes the behavior of clearWithDraw to keep the alpha channel if we should not be clearing it. Bug: angleproject:2667 Change-Id: Id49c1d2ca30ecb5b5bdd8abe00825a6210cfacf6 Reviewed-on: https://chromium-review.googlesource.com/1106052 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Luc Ferron <lucferron@chromium.org>