Need to improve the cutting os S-walls at intersections with branch cuts

[plonghi]

A very minor thing is the cutting of S-walls at branch cuts, it's not super-precise, and this even causes issues in very special cases, for the detection of mutual intersections of S-walls.
We could add a point where the S-wall intersects a cut, and split the wall at such a point, to solve this problem. This is not urgent.