rectile.icn: Program to extract portion of image

This program extracts a fixed rectangle from the images given on the
command line.

The supported options are:

     -x i    x coordinate of upper-left corner of rectangle; default 0
     -y i    y coordinate of upper-left corner of rectangle; default 0
     -w i    width of rectangle; default 64
     -h i    height of rectangle; default 64
     -p s    prefix for name of saved file; default "rect_"; may be
             "", in which case the input file is overridden.