Vulkan: Fix crash with deferred clears and MSRTT The following scenario was mishandled: - MSRTT draw with an unresolve operation (i.e. has two subpasses) - Deferred clear - Flush deferred clear with MSRTT framebuffer not needing unresolve (i.e. has one subpass) Bug: chromium:1178693 Change-Id: If3548e99897d698d61dfafbe9f86193723d06e5a Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2697648 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>