Since we'll also be storing a cache of depth textures as well as
swizzle info, generalize the invalidation methods.

BUG=angleproject:1493
BUG=chromium:638323

Change-Id: I8fef049b01c81483fcf28196866a22d5cfed32a1
Reviewed-on: https://chromium-review.googlesource.com/380036Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>