Implement dirty bits for Framebuffer. The dirty bits set the stage for performance improvements in D3D, but don't actually reduce any of the redundant work just yet. BUG=angleproject:1260 Change-Id: Ib84e6a9b7aa40c37c41790f492361b22faaf4742 Reviewed-on: https://chromium-review.googlesource.com/318730 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tryjob-Request: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>