Vulkan: Edge case test and fix for alpha channel masks The contextVk color mask needed to be updated when we were syncing the state of the framebufferVk to make sure the right mask gets applied in the test described in ClearTest.cpp Bug: angleproject:2685 Bug: angleproject:2597 Change-Id: I575028aad4cd495bf4ec1484cc870940dcde92d5 Reviewed-on: https://chromium-review.googlesource.com/1112038 Commit-Queue: Luc Ferron <lucferron@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>