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.

src/osg/GraphicsContext.cpp

@@ -128,6 +128,7 @@ void GraphicsContext::decrementContextIDUsageCount(unsigned int contextID)
 GraphicsContext::GraphicsContext():
     _threadOfLastMakeCurrent(0)
 {
+    setThreadSafeRefUnref(true);
     _operationsBlock = new Block;
 }