evergreeng: respect linewidth state, use integer widths only

evergreeng: respect linewidth state, use integer widths only

Discard fractional bits from linewidth.  This matches the nvidia
closed drivers, my reading of the OpenGL SI and current llvmpipe
behaviour.

It looks a lot nicer & avoids ugliness where lines alternate between n
and n+1 pixels in width along their length.

Also fix up r600g to match.