support non-rectangular entity sizes in distance calculations

[Grokmoo]

In distance_finder and in the dist_squared method in path_finder, it currently assumes all targets are rectangular and uses a fast method to check nearest point to axis aligned rectangle distance.

If a prop or transition was non square, this shouldn't cause much of a problem, but it would be a bigger issue with actors in combat since you would be able to hit the "corners" of entities even if the corner is not part of their size definition.