Allow placing ghosts of buildings that clash with the terrain.

Allow placing ghost buildings in places that are clashing with or floating above the terrain and differentiate them with red colour. This way it's possible to plan the layout of a building and then plan the necessary earthworks. It also helps with planning connections to underground pipes.

In a similar vein, it would be useful to place ghost buildings that collide with existing buildings. It would also be nice if vehicles could drive through ghost buildings as if they weren't there (because, really, they aren't).