anv: Implement "pop-free" clipping

anv: Implement "pop-free" clipping

This is the preferred clipping mode since it doesn't mean your points
disappear the moment part of the point crosses over the edge of the
viewport and that lines have weird endpoints at viewport edges.  We've
just never bothered to hook it up until now.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>