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>
8 files changed