Before validating the texture format, verify that the texture level already
exists in glTexSubImage calls.

BUG=602737

Change-Id: I0d83c7d43c7b358abdb59583dc83265df70c13b5
Reviewed-on: https://chromium-review.googlesource.com/411361
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>