Reset: given FGScenery a shutdown method.

Forces OSG branch references to be dropped.

src/Scenery/scenery.cxx

@@ -269,6 +269,16 @@ void FGScenery::init() {
     sgUserDataInit( globals->get_props() );
 }
 
+void FGScenery::shutdown()
+{
+    sgUserDataInit( NULL );
+    
+    scene_graph = NULL;
+    terrain_branch = NULL;
+    models_branch = NULL;
+    aircraft_branch = NULL;
+}
+
 
 void FGScenery::update(double dt)
 {

src/Scenery/scenery.hxx

@@ -61,6 +61,7 @@ public:
 
     // Implementation of SGSubsystem.
     void init ();
+    void shutdown ();
     void bind ();
     void unbind ();
     void update (double dt);