Several fixes to fit the map to the latest version of ZS.

Changes:

- Further optimization.
- Removed overlay.
- General increase in difficulty.
- Visual improvements.
- Removed the "radiation" trigger_hurt near the big zombie spawn.
- Replaced the spawns.
- Opened up some of the easy win spots with more passages.
- Flooded area now has a "drain" which acts like a one-way entrance.
- Fixed a prop or two not actually being compiled.
- More potential barricading spots.
- Filtered most breakable brushes to zombies only.
- Other changes in favour or increasing difficulty.
- Lowered water in flooded area.
- Placed a couple ammo boxes.
- Placed a stunstick, axe, sawblade, battery and 3/4 stones in various areas.
- Boss zombie will only spawn at the spawn further away from the bunker.