Modified FlightGear/src/Scenery. The tile loader thread no longer adds

Modified FlightGear/src/Scenery.  The tile loader thread no longer adds
a newly loaded tile to the scene graph.  Instead it puts it in a queue
for the tile manager.  I've used your counter_hack to check the loaded
queue and add any tiles to the scene graph.  I was playing around with
the counter_hack so there might be some commented out code, etc.  I also
changed some SG_DEBUGs to SG_INFOs so I could track the tile loading.