Some non-renderable texture formats remain unimplemented.

BUG=angleproject:1932

Change-Id: Id206432d6e26a70fc0e84478a4e43e9eefadcf2f
Reviewed-on: https://chromium-review.googlesource.com/491948
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>