autotile.icn: Program to produce tile from XBM image

This program creates a tile of a specified size by processing an
XBM image file.  The tile grid is "laid over" the image to form squares.

The non-white pixels in each square of the image are counted. If the
percentage of non-white pixels exceeds a specified threshold, the
corresponding bit in the tile is set.

The supported options are:

     -h i    tile height, default 32
     -w i    tile width, default 32
     -t r    threshold, default 0.50