This should speed up clearUnclearedActiveTextures considerably.
It was showing up as a hotspot when running the aquarium demo with
the passthrough command decoder.

Also rename the complete textures mask in gl::State to an active
textures mask, since it includes incomplete textures.

BUG=angleproject:2188

Change-Id: Idf020fc49c1e74f17a8005c3b88516829767b84c
Reviewed-on: https://chromium-review.googlesource.com/722421Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>