Fix farm greenhouse floors. Add metal floors to grain bins.

[haveric]

Summary

Bugfixes "Fix farm greenhouse floors. Add metal floors to grain bins."

Purpose of change

The grain bins/silos should have a metal floor/concrete instead of dirt (I chose metal floor to be consistent with other silos currently, although there probably should be concrete silo variants).

Farm greenhouses didn't get floor set for some tiles, which causes columns to sometimes spawn there and roof tiles to not exist (although roofs on the farm variants still suck and I don't have a good way to fix that as I couldn't easily override a roof location with the multiple variants)

Describe the solution

Define better floors

Describe alternatives you've considered

None

Testing

Loaded up a new world and spawned in the farm specials. Saw correct floors.

Additional context

None