Supress failures in RGB->L CopyTex[Sub]Image calls on Intel OpenGL. BUG=angleproject:1113 Change-Id: Ie16a28046546fad2ff1c1af35ca67c434b9f5216 Reviewed-on: https://chromium-review.googlesource.com/296481 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>