Internal dist and mindist for iterators

[Major Cooke]

As graf once said, having this feature would be a win for performance. So here's what I'm thinking. 3 new parameters:

- Pointer: The actor to be the center of checking for distance. Defaults to null. If null, ignores the mindist and maxdist parameters.
- Mindist: Just like A_JumpIfTargetInLOS's. If the actor is closer than this, the iterator will skip it.
- Maxdist: Any actors outside this range will be skipped.

[krokots]

An ActorIterator within a radius of some actor? That would be awesome, I use ActorIterators all the time, and with Distance2D inside.

[Major Cooke]

All iterators if applicable.