Make an OSG file reader for .stg files.
New class flightgear::SceneryPager, which is a subclass
osg::DatabasePager to handle explicit delete requests.
Modify FGNewCache, FGTileEntry, and FGTileManager to use
SceneryPager. Mostly this involved removing the queues that talked to
FGTileLoader.
Calculate accurate tile timestamps from the time they are traversed in
the cull stage (which means that they are visible) instead of updating
them periodically.
Replace tile entry transform and range node with one LOD node
