draw/clip: don't emit so many empty triangles

draw/clip: don't emit so many empty triangles

Compress empty triangles (don't emit more than one in a row) and
never emit empty triangles if we already generated a triangle
covering a non-null area. We can't skip all null-triangles
because c_primitives expects ones that were generated from vertices
exactly at the clipping-plane, to be emitted.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>