Makes field of vision and AoE square.

Next beta will probably feature an update to ActorFOV that uses the (CPU hungry) FOV calculations to creation distance maps for all actors at a low cost.

A distance map is an in memory map for each actor that stores a value equal to "game.turn + LOS_radius - distance from source".
Since we can compute it with FOV it's nearly free to do so, and we can use it for fun things:

• Easy pathfinding, just take the value at your coord on your target's distance map, and check all directions for a value higher than yours, if there is, go there. You'll thus "follow the track" of your target, and get lost if it teleports away (which is good)
• Easy fear: do the same but for lower value and you will naturally flow to unseen areas

This feature is kinda like a follow-by-sent which might not be logical for all monsters though. You could also say it follows tracks left on the ground or something like that.

There will be a new "move_dmap" AI that does just that.

I think I'll use it on most T4 monsters.

Comments, improvements?