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>