Add image enqueue commands to front end and pass-through back end.

Bug: angleproject:6015
Change-Id: I133e84020975679fafd29432c965cd19f086d2b4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2940837Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: John Plate <jplate@google.com>