Reads two portable anymaps as input.
Performs the specified arithmetic operation,
and produces a portable anymap as output.
The two input anymaps must be the same width and height.

The arithmetic is performed between corresponding pixels in the two
anymaps, as if maxval was 1.0, black was 0.0, and a linear scale in between.
Results that fall outside of [0..1) are truncated.

The operator
-difference calculates the absolute value of
pnmarith -subtract pnmfile1 pnmfile2, i.e. no truncation is done.

All flags can be abbreviated to their shortest unique prefix.