Add an option to flip between x-y and x-z plane

[xkisu]

The current forced x-y plane from the last commit does not work for generating tiles of models on the x-z plane. To do that it requires reverting to before a62fbda

My suggestion is to add a checkbox somewhere that lets you flip between the 2 planes. If I get around to it I might do it myself and make a pull request :)

[xkisu]

My commits are a bit of a mess (blame github web client for not letting you edit several files at once), but I added a bool to all the applicable places to switch between to 2 planes. You can see my fork here -https://github.com/xkisu/unity-wave-function-collapse

I just did what I needed to to make it work for my project so it's kinda rough and probably not suitable for a pull request, but I think it shows what I mean :)