Structure export "export natural terrain" - does not export terrain under floor tiles etc. #119

Been working on some custom scenario vaults for VFEA, and I was hoping I could have more control over the natural terrain within the export zone, but have run into an issue. When you tick the "export natural terrain" option, it only respects terrain that is fully exposed, which poses a couple problems with regards to level design, and subsequent world gen.

For instance, I have a vault set inside a mountain cave system, so it is important that the terrain underneath the floor tiles is also carried over, as it being soil, sand or anything else that happens to be in those areas on generation, just doesn't make any sense, when it is supposed to be solid rock.

I have also run into issues while playing with the included vaults from VFEA in general, as I play with the geological landforms mod, and some maps, will have a lake in the center. This means, that underneath all the floors and walls, there is actually water, which means you can't place any lights on the walls, cause they are not supported, and also means that you uncover a bunch of water if you try and redecorate the flooring. The same issue obviously persists in any other water/marsh rich areas and similar, so it is a rather common problem. This also causes issues when trying to fix the power grid, as you can't build floors underneath walls, so most of the walls become barriers.

At first, this means that I thought the default vaults just didn't use the "export natural terrain" option at all, and that doing so in my own designs, would remedy these game breaking issues. However, this is sadly not the case, so it would be really nice if this could be addressed.