scenery: Only run SGModelData in the topmost model.

It looks like that we loose the nasal script stored
in the property tree when this callback is issued
multiple times.

This should fix
 http://code.google.com/p/flightgear-bugs/issues/detail?id=714

simgear/scene/model/SGReaderWriterXML.cxx

@@ -218,7 +218,9 @@ sgLoad3DModel_internal(const SGPath& path,
     SGSharedPtr<SGPropertyNode> prop_root = options->getPropertyNode();
     if (!prop_root.valid())
         prop_root = new SGPropertyNode;
+    // The model data appear to be only used in the topmost model
     osg::ref_ptr<SGModelData> data = options->getModelData();
+    options->setModelData(0);
     
     osg::ref_ptr<osg::Node> model;
     osg::ref_ptr<osg::Group> group;