Reads two portable anymaps as input.
Convolves the second using the first,
and writes a portable anymap as output.

Convolution means replacing each pixel with a weighted average of the
nearby pixels. The weights and the area to average are determined by
the convolution matrix.
The unsigned numbers in the convolution file are offset by -maxval/2 to
make signed numbers, and then normalized, so the actual values in the
convolution file are only relative.

Here is a sample convolution file;
it does a simple average of the nine immediate neighbors, resulting
in a smoothed image:

P2
3 3
18
10 10 10
10 10 10
10 10 10

To see how this works, do the above-mentioned offset: 10 - 18/2 gives 1.
The possible range of values is from 0 to 18, and after the offset
that's -9 to 9. The normalization step makes the range -1 to 1, and
the values get scaled correspondingly so they become 1/9 - exactly what
you want.
The equivalent matrix for 5x5 smoothing would have maxval 50 and be
filled with 26.

The convolution file will usually be a graymap,
so that the same convolution gets applied to each color component.
However, if you want to use a pixmap and do a different convolution to
different colors, you can certainly do that.

At the edges of the convolved image, where the convolution matrix would
extend over the edge of the image,
**pnmconvol**
just copies the input pixels directly to the output.