subtract method
Implementation
CSG subtract(CSG csg) {
final a = Node(clone().polygons);
final b = Node(csg.clone().polygons);
a.invert();
a.clipTo(b);
b.clipTo(a);
b.invert();
b.clipTo(a);
b.invert();
a.build(b.allPolygons());
a.invert();
return fromPolygons(a.allPolygons());
}