Better lighting / visibility display

[elliot-nelson]

As the map grows, exploration shouldn't remove all fog of war forever. Add some kind of sight- or room-based fog.

(This serves multiple gameplay purposes: one, less visibility is always more scary. Two, having rooms you can't see be hidden allows you to get away with more, you can move things around under the player and disorient them when appropriate. Three, if moving around shows and hides tiles and rooms there's more visually going on and the game feels more immersive overall.)