Added a Refrenced::getGlobalReferencedMutex, and OpenThreads::ScopedPointerLock() and use of this in add/removeParent() codes

to avoid threading problems when using atomic ref counting.

include/osg/Referenced

@@ -70,11 +70,14 @@ class OSG_EXPORT Referenced
 
         /** Get the mutex used to ensure thread safety of ref()/unref(). */
 #if defined(_OSG_REFERENCED_USE_ATOMIC_OPERATIONS)
-        OpenThreads::Mutex* getRefMutex() const { return 0; }
+        OpenThreads::Mutex* getRefMutex() const { return getGlobalReferencedMutex(); }
 #else
         OpenThreads::Mutex* getRefMutex() const { return _refMutex; }
 #endif
 
+        /** Get the optional global Referenced mutex, this can be shared between all osg::Referenced.*/
+        static OpenThreads::Mutex* getGlobalReferencedMutex();
+
         /** Increment the reference count by one, indicating that 
             this object has another pointer which is referencing it.*/
         inline void ref() const;