The bug was with the readPixels not using the rowPitch, and not really
with the support of npot.

Bug:angleproject:2413

Change-Id: I09c0d87768bc29d3beb452ae83996cd4d2b4a600
Reviewed-on: https://chromium-review.googlesource.com/971830Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Luc Ferron <lucferron@chromium.org>