Connects the holes of pwh with its outer boundary. This is done
by locating the topmost vertex in each hole in the polygon with holes
pwh, and connecting it by a vertical segment to the polygon
feature located directly above it (a vertex or an edge of the outer
boundary, or of another hole). The function produces an output
sequence of points, which corresponds to the traversal of the vertices
of the input polygon; this traversal starts from the outer boundary
and moves to the holes using the auxiliary vertical segments that
were added to connect the polygon with its holes. The value-type
of oi is Kernel::Point_2.

Precondition

The input polygon with holes pwh is bounded
(namely it has a valid outer boundary).