Refactor redefineImage and track dirty images properly. Several issues showed up in testing with WebGL: * Images should only be forcefully re-defined when there is no data to upload. * After an image is marked dirty, a later call to subImage would cause assertion failures because the texture storage would try to verify that the image was not dirty, don't try to copy directly to storage in this case. BUG=angleproject:1635 Change-Id: I9e5d83850d743b7d4d2db938312ee5c35a3a79ee Reviewed-on: https://chromium-review.googlesource.com/527348 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>