Repath if new building revealed

[Shadowen]

Make the pathfinder re-run if an enemy building is revealed to be in the way.

[Shadowen]

http://theory.stanford.edu/~amitp/GameProgramming/MovingObstacles.html#path-splicing

[Shadowen]

719eaa3 adds the new buildings to the underlying pathing graph, but affected paths should be notified.