Add visibility restrictions

[finnbear]

• [ ] Terrain
• [ ] Structures (suggested by St. Elmo's Fire)

This would add depth to the game, by allowing strategies that involve hiding behind the cover of something.

[caibear]

I tried this once on the client, and while somewhat buggy and slow it kind of worked.

I think that the server calculating visibility could have a major performance overhead, and without server validation using a modified client to cheat would be possible.

Note: The client currently displays all information sent to it and thus cheating via increased visibility is impossible.