ETC1 data doesn't need to be fully decoded before encoding to BC1. Saves
~50% computation.

BUG=angleproject:1285

Change-Id: I0cb479a08159ccd0472e5fdf40dd84323151883c
Reviewed-on: https://chromium-review.googlesource.com/361553
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>