Metal: Support GL_ANGLE_provoking_vertex This allows some users to get improved performance while using flat attributes by avoiding index buffer rewrites. Bug: chromium:1340620 Change-Id: Ic55dec27e51d8929d6781758104632defc6861c3 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3738260 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Commit-Queue: Jonah Ryan-Davis <jonahr@google.com> Auto-Submit: Geoff Lang <geofflang@chromium.org>