Possible PGM_BIGGRAYS writing bug.

Integrate RLE-compression into pnmtogif.

Add pbmerode, and any contributed stuff lying around in email.

Add pnmtopng.

What happened to pnmtojpg?

Add an auto-transparent mode to pnmtogif.

Add ppmtocmyk4, based on ppmtorgb3 and/or ppmtoyuvsplit..

The Converter Generator reorganization: all filters become three library
routines, one to initialize, one to handle each line, and one to finish off.
This will make it possible to generate self-contained pipelines on demand,
and they should run substantially faster without all that I/O.