In preparation for adding blit functionality.

Bug: angleproject:3200
Change-Id: I77b6b6cbe90b26ee23c25edebaa2b130f3c2987a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1641626
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>