pnm: rewrite PAM header decoder

The new decoding logic does not require any unbounded length allocations.

Behavioral changes:
* The decoder now rejects leading + on numbers, to match PNM.
* The length of custom TUPLTYPEs now has a hard limit. This does not
  affect realistic images, and PnmDecoder only accepts a short list of
  known TUPLTYPEs anyway.  (Without a hard limit, the PAM header
  decoding logic would require memory limits to be specified in
  advance.)

Author: mstoeckl