New procedure inside_4, used in set_max_speed in the same way as

procedure inside was used in previous commits, but inside_4 is based
on polygon_contains_point.

In procedure set_max_speed, extend the search for radius beyond the
value provided by polygon_point_dist_2d. We are now certain that
ind_extr + radius is outside. We recover the value of radius that was
computed by inside assuming no degeneracy.