External textures are often not renderable for the CPU readback path.
Instead, copy them into a renderable texture and then read them back.

BUG=906724

Change-Id: I61bd0a9d51c4b24e1d285b5f1883e01ccb53ebea
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1680053
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>