This module fills a set of polygons (e.g. created using the
module \verb+readpoly+) with the gray level 0 (black).
The result is bitmapped in a Cimage.
Pixels outside any polygon are set to the gray level 255 (white).
The user has to specify the size of the Cimage, which must contains of course
all the polygons. The defaut size is $256 \times 256$.
Such function is useful to generate the first entry of the 
module \verb+movepoly+.
It has a pseudo-inverse transform called \verb+emptypoly+.

