Don't initialize textures on SubImage calls if the whole image will be filled. BUG=angleproject:2107 BUG=angleproject:2196 Change-Id: I3e7e78bfc64f1683921af2e48fe20c7be0b85af3 Reviewed-on: https://chromium-review.googlesource.com/734228 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>