Rather than try to read back compressed data using ANGLE_get_image
(which does not yet support compressed formats), track a shadow copy
of the compressed image and store it like client data.

Test: Mid-execution capture and replay of TRex frames using ETC1
Bug: angleproject:4036
Change-Id: Id15be60f7444cc2e1356e76ff18069c42e552663
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2015818Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Cody Northrop <cnorthrop@google.com>