A numeric value giving the perimeter length of the window.
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.