This fills the field \texttt{boundary} of a \texttt{Shape} issued from a tree
(see \texttt{flst\_boundary}). It is a list of 2-D points with integral
coordinates. The last point is equal to the first one if the shape is
closed. If the shape is open, the two extremal points are at the border of the
image. The shape is followed in counterclockwise direction, so that the shape
is on left hand side and the exterior on right hand side.
\index{level line}
\index{interpolation!nearest neighbor}
\index{Fast Level Set Transform} 
