swr: [rasterizer core] supply proper clip distances to point sprites

swr: [rasterizer core] supply proper clip distances to point sprites

Large points become pairs of triangles when rasterized, so we must feed
it three clip distances, one for each vertex.

The clip distance is not subject to sprite coord replacement, so there's
no interpolation of it. We just take its value and put it in the "z"
component of the barycentric-ready plane equation.

(We could also just cull it at an earlier point in time, but that would
require larger changes.)

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Tim Rowley <timothy.o.rowley@intel.com>