This is a prerequisite to support incomplete textures.

Bug: angleproject:2536

Change-Id: Ica40bbd185a67253f457148007b08f6735da788c
Reviewed-on: https://chromium-review.googlesource.com/1050308Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Luc Ferron <lucferron@chromium.org>