GL: Allow BlitGL to copy from external textures. BUG=967410 Change-Id: I503dda6f2493b56123a8bd369e2f1305971abb4e Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1364110 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Commit-Queue: Geoff Lang <geofflang@chromium.org>