Passes all dEQP tests related to etc1.

Change-Id: I3ab2d51b7664cb2411fdc3247ff65a9e3258185a
Reviewed-on: https://chromium-review.googlesource.com/299340Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>