Currently this method doesn't handle partial offsets.
Fixes dEQP tests in the texture specification group.

BUG=angle:899

Change-Id: I20160e49a46957b4f6cf96f529ab4ea7ddf7b343
Reviewed-on: https://chromium-review.googlesource.com/244091Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>