Notes on what is required to make grf4 handle hidden lines.

1. The polygon intersect code must be modified so that as polygons
   are split, it still works:

{
	for (p1=list; p1; p1=p1_next)
	 { p1_next = p1->next;
	   checkRemainingPolys (p1, p1_next);
	 }
	checkRemainingPolys(p1, prest)
	 { for (p2=prest; p2; p2=p2_next)
	    { p2_next = p2->next;
	      COMPARE p1 and p2.
	      if (p2 split)
	       { add p2_a into list before p2;
		 don't do anything else -- we will catch p2_b later
	       }
	      if (p1 split)
	       { add p1_a into list before p1;
		 checkRemainingPolys (p1_a, p2_next);
	       }	      
	    }
	 }
}

2. How do we check the polygons in the first place?

{
	if (the polygons don't intersect)
	 { return;
	 }
	do gross depth test. 
	if this is conclusive, return the appropriate depth

	if (polygons are not parallel)
	 { compute intersection line.

	   if (intersection zone intersects the line)
	    { split the polygon which supplied the intersection points.
	      if (both polygons supplied the intersection points)
	       { split either polygon;
	       }
	      call the polygon obscure code for both split polygons
	      with the other polygon.
	    }
	   else
	    { do a depth test in the zone center.
	      obscure the losing polygon with the winner.
	    }
	 }
	else
	 { do a depth test in the zone center.
	   obscure the losing polygon with the winner.
	 }
	
}



