Added roof to two shacks

[PatrikLundell]

Summary

None

Purpose of change

Add explicit roofs to two shacks in farm_nested.json (one of which is used only in trails, bizarrely enough).

Describe the solution

Add roof chunks and nest them from their respective building chunks.

Describe alternatives you've considered

Testing

• Debug spawn horse farms until a greenhouse is generated, teleport to the greenhouse, and examine it.
• Debug spawn fire lookout towers, teleport to their adjacent trail part, examine the part, and repeat until the shed is generated.
  
  
  
  

Additional context

Getting the smaller shed to generate wasn't trivial. The fire lookout generates it, fortunately, and we all know the trail connects fire lookout towers to farms...

[LovamKicsiGazsii]

It's so cool that nested mapgen can span multiple levels now.

[PatrikLundell]

We're having some teething problems with the 3D support, but it seems manageable.