New function polygon_insert_vertex_after

This function insert a vertex inside the double linked list and keep the
whole data structure of the polygon consistent. This function assume the
given vertex is part of the polygon.

Signed-off-by: Celelibi <celelibi@gmail.com>
Use the function polygon_insert_vertex_after

This function keep the whole data structure consistent and should be
used instead of playing around with the linked list of vertices. This
resolves the bug of filling a polygon after cutting an edge.

Signed-off-by: Celelibi <celelibi@gmail.com>