Rounding to power-of-two dimensions

First, thanks for publishing sheep! It was heartening to see other people working on asset pipeline tooling for games in Rust.

In a tool my team is working on, we need to pack images into square power-of-two textures for some devices, preferring to waste a little space to get a square image instead of a perfectly packed non-square image.

When using Sheep's Max Rects packer with five 128x128 textures and a max size of 1024x1024, it produces a 128x640 texture like this:


We'd prefer that it instead produce a 512x512 square texture with wasted space. On some platforms, would have to expand a 128x640 texture into a 1024x1024 texture, which is more wasteful!

We've found [some other tools](https://github.com/soimy/maxrects-packer) that mention the problem of PO2 textures with maxrects, but I'm not familiar enough with packing algorithms to know if it's feasible to enforce exactly.