which utilises a global recursive mutex that is dedicated to manage Observer and ObserverSet.
The new global mutex for observers avoids problems with deadlocks that were occurring previously when
an osg::Refenced object was being deleted at the same time as on osg::ObserverNodePath.
the extra capability of making it possible for Observers to assume ownership of a object that would otherwsie be deleted.
Added a thread safe ref_ptr<T> observer_ptr<T>::lock() method for robust access to an observed object. This
makes observer_ptr<> more equivilant to boosts weak_ptr.
