D3D11: Reduce overhead of clearing unused textures. 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: I92f11cf63542c55bb3adbbb8276a54bcfe848f61 Reviewed-on: https://chromium-review.googlesource.com/277118 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>