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

