For V3D, the HW will interpolate slightly differently along the shared
edge of the trifan. The conformance tests manage to catch this in the
nearest_consistency_* group. To get interpolation to match, we need the
last vertex of the triangle to be shared.
I first tried implementing draw_rectangle to do triangles instead, but
that was quite a bit (147 lines) of code duplication from u_blitter, and
this seems much simpler and less likely to break as u_blitter changes.
Fixes dEQP-GLES3.functional.fbo.blit.rect.nearest_consistency_* on V3D.
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
