Peter Wang is me.

Thanks to Martijn Versteegh for allpng.c, from which I took the actual
routine which saves 15 / 16 bpp images.

The following people reported or fixed bugs or did something else
(assuming I didn't forget):

- Dave Dribin
- Lennart Steinke
- Chris Graham
- Robbert Haarman
- Ceniza
- Nicolas Lemal
- Jon Rafkind
- Emmanuel Anne