Optmization: Lazy allocation of ArrayLists in
SpacePartitioningGrid.GridSquare to save memroy.
Bugfix: Non-weak reference of otherDEF contritubed in OverworldSystem
not being garbage collected.
Improved null-checking in CollidesWithTerrain and TunnelRailed.
Cleanup of unneeded OverworldSystem deactivation.
Removed TR.getAltitudeMap() in favor of OverworldSystem.getAltitudeMap()
Removed TR.getBackdropSystem() in preference to Game.getBackdropSystem()
Removed hard references to segment placeholders in
CollidesWithTunnelWalls and TunnelRailed to prevent memory leaks.