Create a polygonal clip region. You must call endclip() after
        you completed drawing. See also the polygon method.

def clip_polygon(self, points):
        """Create a polygonal clip region. You must call endclip() after
        you completed drawing. See also the polygon method."""
        self.gsave()
        self._path_polygon(points)
        self.__clip_stack.append(self.__clip_box)
        self.__clip_box = _intersect_box(self.__clip_box, _compute_bounding_box(points))
        self.clip_sub()