Generate an error if no XFB varyings are in use GLES specifies an error if BeginTransformFeedback is called when no binding points would be used. BUG=angleproject:2184 TEST=angle_end2end_tests Change-Id: Ie4489b5ba63885e718dafdcdaacc02b603959be3 Reviewed-on: https://chromium-review.googlesource.com/719136 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>