Refactored the osg::Observer to introduce a new bool Observer::objectUnreferenced(void*) method that adds

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.

include/osg/Observer

@@ -0,0 +1,40 @@
+/* -*-c++-*- OpenSceneGraph - Copyright (C) 1998-2006 Robert Osfield
+ *
+ * This library is open source and may be redistributed and/or modified under
+ * the terms of the OpenSceneGraph Public License (OSGPL) version 0.0 or
+ * (at your option) any later version.  The full license is in LICENSE file
+ * included with this distribution, and on the openscenegraph.org website.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * OpenSceneGraph Public License for more details.
+*/
+
+#ifndef OSG_OCCLUDER
+#define OSG_OCCLUDER 1
+
+namespace osg {
+
+/** Observer base class for tracking when objects are unreferenced (there reference count goes to 0) and are being deleted.*/
+class Observer
+{
+public:
+    virtual ~Observer() {}
+
+    /** objectUnreferenced(void*) is called when the observed object's referenced count goes to zero, indicating that
+      * the object will be deleted unless a new reference is made to it.  If you wish to prevent deletion of the object
+      * then it's reference count should be incremented such as via taking a ref_ptr<> to it, if no refernce is taken
+      * by any of the observers of the object then the object will be deleted, and objectDeleted will in turn be called.
+      * return true if the Observer wishes to removed from the oberseved objects observer set.*/
+    virtual bool objectUnreferenced(void*) { return false; }
+
+    /** objectDeleted is called when the observed object is about to be deleted.  The observer will be automatically
+      * removed from the observerd objects observer set so there is no need for the objectDeleted implementation
+      * to call removeObserver() on the observed object. */
+    virtual void objectDeleted(void*) {}
+};
+
+}
+
+#endif