In the iOS simulator we use glReadPixels to transfer rendered pixels to
the IOSurface. We need to account for the possibility of the row stride
not being equal to the width, because that happens sometimes. It's
currently causing corrupt rendering in Google Maps on WebKit.

Bug: angleproject:4611
Change-Id: I553d65cb0d0bf922e855ea50089904807dd39118
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2181676Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>