Use the last triangle vertex for flat shading on OpenGL. Bug 21559270 Change-Id: I4c2b5eb7076cc459422fbbb397a684c19bd643c2 Reviewed-on: https://swiftshader-review.googlesource.com/3372 Reviewed-by: Nicolas Capens <capn@google.com> Tested-by: Nicolas Capens <capn@google.com>