This module draws a set of Curves (integer valued) on a Cimage.
The image dimensions are computed to fit exactly the bounding box
of the Curves structure.
With \verb+-l+ option, the successive points are linked by a straight 
segment. In this case, the draw mode (\verb+-d+ option) allows 
to choose between a simple plot ($d=0$ : black lines) and a 
contrasted plot ($d=1$ : black lines surrounded by white).
