Skyline packer

I've read about skyline heuristics given in [2011](https://www.sciencedirect.com/science/article/abs/pii/S0377221711005510) and [2016](https://www.sciencedirect.com/science/article/pii/S0305054816302982) works and they both work by trying to achieve as low height as possible given a width of a spritesheet.

1. I'm not sure how to adopt this to output multiple spritesheets, since their approach relies heavily on being able to shuffle the input. One possible approach is trying to fit it into one spritesheet, then if it fails, splitting the input randomly into two parts and trying to fit it into two spritesheets and so on. I'm not sure this is an optimal approach and would like to hear your thoughts.

2. I'm not sure how to choose the width of a spritesheet, I don't really like `maxrects` approach of giving it a default value since in it will just produce a long horizontal stripe of sprites and that doesn't feel optimal (maybe it's really efficient or doesn't make a difference at all, I don't know). So I propose to make some kind of algorithm on top of skyline (and maybe maxrects too) that tries to determine optimal width and height. Again, I'd like to hear your thoughts.