Implements workarounds for:
 - The pack state making rows overlap in memory, which causes crashes on
   some drivers.
 - The driver adding an extra last row padding when checking if the
   pixel pack buffer is large enough for the readPixels.

BUG=angleproject:1512

Change-Id: I120ff58649bb523e8b01da6ef03d8fcadaf076b2
Reviewed-on: https://chromium-review.googlesource.com/388029
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>