MaxRectBinPack should be multi-threaded

[stefangordon]

This is one of the slowest parts of the processing right now because the bin packing is single threaded.

A simple option might just be spawning the bin packing off in a task and letting the app continue on to the next texture tile. I'm not sure if this is possible until we remove System.Drawing, as I think creating graphics objects across multiple threads fails.

[stefangordon]

We can also look for a more effective way to predict the final tile size. Right now the bin packer starts with a small bin and keeps doubling the size until all the rects fit. This may mean doing the bin packing several times for each image. It is critical that the output textures be certain dimensions though, so just using a giant bin and cropping afterwards won't work.