Made Refernced::setThreadSafeReferenceCounting(bool) a virtual then overrode

this in various scene graph classes to ensure that the scene graph gets
updated as well as the objects that the initialial call is made from.

include/osg/Drawable

@@ -278,6 +278,9 @@ class OSG_EXPORT Drawable : public Object
         */
         virtual void compileGLObjects(RenderInfo& renderInfo) const;
 
+        /** Set whether to use a mutex to ensure ref() and unref() are thread safe.*/
+        virtual void setThreadSafeRefUnref(bool threadSafe);
+
         /** Resize any per context GLObject buffers to specified size. */
         virtual void resizeGLObjectBuffers(unsigned int maxSize);