Quantization and efficient frame encoding

[photopea]

Thanks to your library, I implemented GIF opening and saving into my online editor Photopea.com. It can convert between APNG and GIF and also minify GIFs and APNGs (by shrinking a color palette). In order to make an efficient GIF encoder, color quantization (256 colors) and efficient frame "cropping" is necessary.

I made a library UPNG.js for decoding and encoding PNG (APNG). There is a function UPNG.encode.compress(), which performs quantization and frame cropping on 8-bit RGBA images. Its output can be used as an input for omggif.

Maybe you could mention UPNG.js in Readme, so your users could combine both libraries to make an efficient encoder :)