This function computes the perimeter (length of the boundary) of the
  window w. If w is a rectangle or a polygonal window,
  the perimeter is the sum of the lengths of the edges of w.
  If w is a mask, it is first converted to a polygonal window
  using as.polygonal, then staircase edges are removed
  using simplify.owin, and the perimeter of the resulting
  polygon is computed.