Special Forces Group 2 uses glCopyImageSubData() to copy compressed
texture data, which requires special handling for ANGLE's frame
capture since compressed texture data can't be read back.

To support this, when glCopyImageSubData() is called with textures using
compressed formats, a new mCachedTextureLevelData entry will be created
for the destination texture which is a copy of the source texture data.

Note that this implementation only supports copying the full level data.
http://anglebug.com/5604 will track adding support for partial level
copies if an app requires that in the future.

Bug: angleproject:5592
Bug: angleproject:5604
Change-Id: Ibce13f91f838fd691269c796fbeb5b9c6133c6aa
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2669955Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Tim Van Patten <timvp@google.com>