Part 1 in a series of changes to perform image copies on the GPU.

Bug: angleproject:2958
Change-Id: I6264a880865c4738c0866f2dc71af63425fc4118
Reviewed-on: https://chromium-review.googlesource.com/c/1370724
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>