Fire buffs, wall damage

Walls can now take damage from explosions and fire.
Damage can be repaired with a weldingtool, if it gets too high the wall will break down to girders.
![wall damage](https://i.imgur.com/Djq5NOJ.png)

Fires can now burn through walls, windows, and (with a high enough temperature) rwalls.
New proc: adjacent_fire_act().  Called on turfs that a fire isn't able to spread to. (walls, floor surrounded by windows, etc.)
![fire damage](https://i.imgur.com/aUeOL2V.png)

Bonus post-fire screenshot: https://i.imgur.com/1VrZiip.png

also fixes a ZAS bug so fire will spread properly.
This way, every wall in the game shares the same list so only 8 images are ever created.
If I did that, every wall in the world could end up with its own image.
efficiency!