Encode and decode PNM image files (TODO: so far only PPM).
- Decode ppm image files.
- Encode ppm image files.
- Encoding & decoding is integrated with afi crate.
- Support all PNM variants like PGM etc.
- Add support for PNM files with multiple frames.
- Multithreaded png loading to load a bunch of files at once.
- Now uses afi 0.7 for API template.