Round explicitly after dithering, for GPUs that don't do it properly themselves.

Round explicitly after dithering, for GPUs that don't do it properly themselves.

This was causing unit test failures in the DitherEffect unit test both on
ATI and nVidia GPUs; Intel also rounds somewhat inaccurately, but much,
much better, so the extra code won't be activated for them.

I think this might be driver-dependent, but we will detect it correctly
in any case.