Include the "pixels" (data) pointer as offset.

Bug: angleproject:5702
Change-Id: Idc9f3ee4c0dbb1cc9308393bbf877d6137598486
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2732789Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Stephen White <senorblanco@chromium.org>