Performance improvements + bugfixes

New update!

1. Zombie performance is significantly improved, thanks to a combination of:

• Making zombie hitboxes/viewboxes rectangles instead of circles, and removing the their "rotation" property
• Limiting the amount of nearby zombies other zombies are aware of
• Only updating complex zombie pathfinding (used for sound-chasing) every half-second instead of every frame

There is still a bit more that can be done but this already makes a huge difference to the game's playability I think!

2. Civilians now correctly flee from zombies (I broke this in a previous update and didn't notice), and pathfind towards the train more aggressively.

3. Zombies can now spawn from multiple directions out of a spawner tile at once

4. A bug whereby dead ends could still be placed in specific circumstances has been fixed.