The X channel wasn't taken into account when calculing that pixel_bytes
field, leading to some inconsistencies.

Bug: angleproject:2597
Change-Id: I135e806518ec633d48c1535a48f305bd4d757887
Reviewed-on: https://chromium-review.googlesource.com/1104960Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Luc Ferron <lucferron@chromium.org>