BlitGL: orphan scratch textures when we are done. This will allow the OpenGL driver to reclaim GPU memory. BUG=angleproject:1512 Change-Id: If063e1ff9f4a544012ca78c37e198054cb4edcac Reviewed-on: https://chromium-review.googlesource.com/388771 Commit-Queue: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>