fix avenger roof.
fix for confusion over distance funciton.
Zombies should spawn without TUs
remove unnecessary include.
Feature addition: Item Stacking in inventory screen.

only items that match can be stacked.
matching conditions:
- is the same item type. (uses the same ruleset)
- is loaded with the same type and amount of ammo. (including none at all)
- is not a live grenade.
- is not a corpse.