D3D11: Minor optimizations to vertex attribute application. Introduce a dirty bit for current value attribs, and try to speed up the check for dirty vertex attribs. This series of small optimizations gives about a 15% improvement on the draw call benchmark for the D3D11 backend with the null driver. BUG=angleproject:1155 Change-Id: Idf560efa3af62776e8bdbdf693f8b06db8792c21 Reviewed-on: https://chromium-review.googlesource.com/666048 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>