u_blitter: Add an option to draw the triangles using an index buffer.

u_blitter: Add an option to draw the triangles using an index buffer.

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>