These will have to be fleshed out in the back-ends.
Also currently uses a single bit for all the bindings, and we can
extend this to more fine-grained updates in the future.

This patch implements top-level updates for texture completeness.
Sampler completeness caches are removed from the Texture class, and
replaced by a cache in the gl::State class. The State class also
keeps a channel binding to the bound textures so it can be notified
when textures might change from complete <-> incomplete.

In future CLs we skip updating back-ends if texture state doesn't
change.

BUG=angleproject:1387

Change-Id: If580b4851303c86f3240e62891f5f6047eefb6a2
Reviewed-on: https://chromium-review.googlesource.com/648053Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>