Cutting holes

[brnkhy]

@david-rhodes can you check this if/when you have time please?
This'll probably be a little hard to merge in as it's a quite big PR but would be nice if we can test&talk about it next week.

[david-rhodes]

• Not all examples updated to use new modules (Drive and Slippy are broken)
• Snap y to zero (MapController) does not seem to be working
  • Use Boulder, Co, for MeshGeneration.unity location and you will see the tiles are too far above the ground plane to see in camera view
• Modifiers need to have a static order (custom inspectors can even reorder as they are added)
  • We could have always benefitted from this, but now that we have even more modifiers, they need to be ordered properly (by us).
• Roofs on terrain are still not straight:

On the up side, there are zero conflicts when trying to pull develop in 👍.

[brnkhy]

@david-rhodes I fixed a small issue with y axis coordinates.
flat roofs should be OK now, you sure you had Flat Roof checked in height modifier?

demos might be totally broken, files are in bad/wrong folders as well. I'll polish this and get ready to merge asap.
snap y is globally broken I guess, I'll check that as well.

[brnkhy]

@david-rhodes I fixed some scenes and module references. should be easier to review now.
I had different plans for modifier hierarchy/order but looks like it'll actually be much better if we switch to a much more restrictive system for that in short term. I have some ideas for that, let's talk whenever you have time.