draw: fix viewport index update to primitive boundaries.
I'd like to reopen the bidding on this change. I've tried changing things to respect flatshade_first to no useful outcome.
My take on this is that same scale and transform have to be applied to the position for all vertices in the primitive. I'm not sure why we'd only apply it on the provoking vertex in the first place.
This fixes piglit arb_viewport_array-render-viewport-2 KHR-GLES31.core.viewport_array.draw_to_single_layer_with_multiple_viewports,Fail KHR-GLES31.core.viewport_array.draw_mulitple_viewports_with_single_invocation,Fail KHR-GLES31.core.viewport_array.draw_multiple_layers,Fail KHR-GLES31.core.viewport_array.depth_range,Fail