Return a MousePos filled with the world position and surf it hit.

def get_mouse_pos(self, window_pos=None):
    """Return a MousePos filled with the world position and surf it hit."""
    window_pos = window_pos or pygame.mouse.get_pos()
    # +0.5 to center the point on the middle of the pixel.
    window_pt = point.Point(*window_pos) + 0.5
    for surf in reversed(self._surfaces):
      if (surf.surf_type != SurfType.CHROME and
          surf.surf_rect.contains_point(window_pt)):
        surf_rel_pt = window_pt - surf.surf_rect.tl
        world_pt = surf.world_to_surf.back_pt(surf_rel_pt)
        return MousePos(world_pt, surf)