This function computes, for each point $x_i$
in the point pattern
X, the shortest distance
$d(x_i, W^c)$ from $x_i$
to the boundary of the window $W$ of observation.
If the window
X$window is of type
"polygonal", then these distances are computed by
analytic geometry and are exact, up to rounding errors.
If the window is of type
"mask" then the distances
are computed using the real-valued distance transform,
which is an approximation with maximum error equal to the width
of one pixel in the mask.