ES3 adds a clause that cube map FBO attachments must be cube complete
to produce complete framebuffers. ES2 doesn't have this clause, but
some (if not all, unverified) OpenGL back-ends don't support these
incomplete cube map attachments.

BUG=angleproject:1259

Change-Id: Idd4564488375b8646dde712e6ce4a158c23020ee
Reviewed-on: https://chromium-review.googlesource.com/318264
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Rafael Cintron <rafael.cintron@microsoft.com>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>