Missing format support

https://flakdeau19.itch.io/high-resolution-pixel-art-textures

These fail

```
pigment64 to-bin --format rgba32 qi_rCt.png
thread 'main' panicked at 'unsupported format (Rgba, Sixteen)', /home/ethteck/repos/pigment64/src/image/png_image.rs:280:18
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```