Improve palette packing algorithm

[ISSOtm]

tl;dr: some simple images end up packing poorly with the current algorithm.

https://github.com/ISSOtm/no-packsaran has some more details, and a link to an implementation of a more efficient packer, if you want.

[asiekierka]

The provided link 404s; it should be https://github.com/ISSOtm/no-packsaran