D3D11: Reduce overhead of clearing unused textures. *re-land with compile fix* We would spend a fair bit of time iterating over the unused textures, setting them to null one-by-one, and updating our cache. We can reduce this time by smarter caching, and skipping unmodified ranges. BUG=angleproject:959 Change-Id: I4de20bc131c4a568108ad670a2ef491cfd4c50ed Reviewed-on: https://chromium-review.googlesource.com/280916 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>