Moves every edge of a counter-clockwise polygon inwards by its own distance (d[i] for edge
p[i]→p[i+1]) and joins the offset lines at their intersections. Edges that collapse (their offset
runs backwards) are removed and the neighbours re-joined, a cheap stand-in for a straight skeleton.
Returns null when nothing valid is left.
Moves every edge of a counter-clockwise polygon inwards by its own distance (
d[i]for edge p[i]→p[i+1]) and joins the offset lines at their intersections. Edges that collapse (their offset runs backwards) are removed and the neighbours re-joined, a cheap stand-in for a straight skeleton. Returnsnullwhen nothing valid is left.