Capture/Replay: Add ANGLE_get_image extension. Will be used for mid-execution capture. Only has a stubbed-out implementation right now. Bug: angleproject:3944 Change-Id: I6ddae07907ecbdbd3be9a5d2d3fcafeb425445e4 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1878888 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Cody Northrop <cnorthrop@google.com>