Plague 3.30

• Changed shadowing algorithm - removed artifacts, nice clean code
• bugfix: shrubs now leave behind burnt shrubs when burnt - not sure when I broke this
• bugfix: AI sims try to pick up guns (again)
• bugfix: AI sims now switch weapons if they run out of ammo (again)
  
• tidied for world.add_sim so that force is used where appropriate, and other checks removed
• added a key (j) so you can see the new shadow maps - I think they are kinda cool  (and they were good for debugging) .