Ensure every scenery model has own SGModelData.

This makes Nasal unload hooks of scenery objects working again.
Previously the same SGModelData instance was used for all objects
which never got destroyed and therefore was not able to call any
unload callback.

simgear/scene/model/modellib.hxx

@@ -93,6 +93,7 @@ public:
     virtual ~SGModelData() {}
     virtual void modelLoaded(const std::string& path, SGPropertyNode *prop,
                              osg::Node* branch) = 0;
+    virtual SGModelData* clone() const = 0;
 };
 
 }

simgear/scene/tgdb/ReaderWriterSTG.cxx

@@ -185,6 +185,14 @@ struct ReaderWriterSTG::_ModelBin {
         std::string fg_root = options->getPluginStringData("SimGear::FG_ROOT");
         sharedOptions->getDatabasePathList().push_back(fg_root);
 
+        // TODO how should we handle this for OBJECT_SHARED?
+        sharedOptions->setModelData
+        (
+            sharedOptions->getModelData()
+          ? sharedOptions->getModelData()->clone()
+          : 0
+        );
+
         return sharedOptions.release();
     }
     SGReaderWriterOptions* staticOptions(const std::string& filePath, const osgDB::Options* options)
@@ -196,6 +204,15 @@ struct ReaderWriterSTG::_ModelBin {
         staticOptions->getDatabasePathList().push_back(filePath);
         staticOptions->setObjectCacheHint(osgDB::Options::CACHE_NONE);
 
+        // Every model needs its own SGModelData to ensure load/unload is
+        // working properly
+        staticOptions->setModelData
+        (
+            staticOptions->getModelData()
+          ? staticOptions->getModelData()->clone()
+          : 0
+        );
+
         return staticOptions.release();
     }