Vehicles now spawn in more interesting ways.

A number of fixes/improvements to how vehicles are spawned:

* Vehicles don't spawn in water (Fixes #1801)
* Shopping carts no longer spawn inside walls (Fixes #2707)
* Vehicles CAN now spawn inside other vehicles - and both will be *extremely* damaged if this happens. Creates the appearance of nasty collisions.
* Vehicles CAN spawn in walls - causing extreme masonry demolition in the process and *heavily* damaging the vehicle. (With current spawn locations, however, this is highly unlikely)

For maximum amusement, someone should follow this up with a PR to make vehicles occasionally spawn along the walls of House squares, making it look like someone swerved into a wall.