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>