Reset: model lib can drop static root.

simgear/scene/model/modellib.cxx

@@ -57,6 +57,11 @@ void SGModelLib::init(const string &root_dir, SGPropertyNode* root)
     static_propRoot = root;
 }
 
+void SGModelLib::resetPropertyRoot()
+{
+    static_propRoot.clear();
+}
+
 void SGModelLib::setPanelFunc(panel_func pf)
 {
   static_panelFunc = pf;

simgear/scene/model/modellib.hxx

@@ -46,6 +46,8 @@ public:
 
     static void init(const std::string &root_dir, SGPropertyNode* root);
 
+    static void resetPropertyRoot();
+    
     static void setPanelFunc(panel_func pf);
     
     // Load a 3D model (any format)