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 `"rectangle"`

or `"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.