Vulkan: Add ContextVk dirty bits. These dirty bits mostly track state needed for the command buffer. It should speed up ContextVk::setupDraw by skipping unset dirty bits instead of using boolean checks. Improves perf slightly on the Vulkan null draw call test. Bug: angleproject:2786 Change-Id: I071ce558c522732a17ad101d097ee4d10740abf1 Reviewed-on: https://chromium-review.googlesource.com/1194300 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Frank Henigman <fjhenigman@chromium.org>