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/277118Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>