Z-Levels: Vehicle Debris spawns on wrong level

Experimental Build 0.C-3227, Commit ddc1f13, Experimental Z-Levels On.

I recently tried my hand at establishing an underground base for my survivor. I also was running the experimental Z-levels (very nice work, Coolthulu). Things went fairly well, but I noticed that when zombies would dogpile on a vehicle above ground and start dismantling it, the debris and assorted parts would spawn on the z-level I was currently on, rather than the z-level the vehicle was on. This kinda wrecked the game I was playing, because it basically meant free vehicle parts I didn’t really have to fight or scavenge for (plus tons of scrap metal and other assorted scrap to craft with).

I keep reminding myself to fix this bug, then I keep forgetting that it exists.
You reminded me at the perfect time - I’m doing z-level bashing related stuff, so I’ll just include it in the PR I’m working on.