Added default setting of use of thread safe ref/unref.

include/osg/DisplaySettings

@@ -36,13 +36,15 @@ class OSG_EXPORT DisplaySettings : public osg::Referenced
         static DisplaySettings* instance();
 
 
-        DisplaySettings()
+        DisplaySettings():
+            Referenced(true)
         {
             setDefaults();
             readEnvironmentalVariables();
         }
         
-        DisplaySettings(ArgumentParser& arguments)
+        DisplaySettings(ArgumentParser& arguments):
+            Referenced(true)
         {
             setDefaults();
             readEnvironmentalVariables();

include/osg/NodeVisitor

@@ -261,7 +261,11 @@ class OSG_EXPORT NodeVisitor : public virtual Referenced
         /** Callback for managing database paging, such as generated by PagedLOD nodes.*/
         class DatabaseRequestHandler : public osg::Referenced
         {
-        public:            
+        public:
+        
+            DatabaseRequestHandler():
+                Referenced(true) {}
+        
             virtual void requestNodeFile(const std::string& fileName,osg::Group* group, float priority, const FrameStamp* framestamp) = 0;
             
         protected: