Passes all dEQP tests related to etc1.

Change-Id: I33f37c44396899d87e9ef19bd7a6e2b2482d232a
Reviewed-on: https://chromium-review.googlesource.com/299342Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>