This adds basic validation for catching CopyTex{Sub}Image calls
whose source and destination textures overlap. It does not yet
implement full support for ES3 types (3D textures, array textures).
BUG=angleproject:1685
Change-Id: I83e7b1998df5575057fed8f99f7ee9970fb38df0
Reviewed-on: https://chromium-review.googlesource.com/425491
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
