/* convert control point representation to
d*x^3 + c*x^2 + b*x + a */
+
+ /* FIXME: we need to do this for the subspline between [start,end],
+ not [0,1] */
dx= s->end.x - s->control2.x*3 + s->control1.x*3 - s->start.x;
dy= s->end.y - s->control2.y*3 + s->control1.y*3 - s->start.y;