Also fixes the unpack buffer offset which must be in pixel instead of
bytes in the shader parameter.

BUG=angle:508

Change-Id: I908460a76c300d96054a02206e1f2e3ad2b8e9d0
Reviewed-on: https://chromium-review.googlesource.com/241440Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Gregoire Payen de La Garanderie <Gregory.Payen@imgtec.com>