Vulkan: Fix clear value for incomplete MS textures * Added initializeContentsWithBlack() for the cases we wish to use opaque black. * Added stageResourceClearWithFormat(), which also takes an initial clear value. stageRobustResourceClearWithFormat() is included as a special case. Bug: angleproject:7151 Change-Id: I32d7a9af53b51499fe46d6e397301744dbf3c236 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3652995 Commit-Queue: Amirali Abdolrashidi <abdolrashidi@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>