Added selective support for thread safe ref/unref such that the rendering

backend now doesn't use thread safe ref counting where multi-buffering exists.
This reduces the overhead of multi-threading.

include/osg/Object

@@ -52,6 +52,8 @@ class OSG_EXPORT Object : public Referenced
             concrete classes and can be constructed.*/
         inline Object():Referenced(),_dataVariance(DYNAMIC) {}
 
+        inline explicit Object(bool threadSafeRefUnref):Referenced(threadSafeRefUnref),_dataVariance(DYNAMIC) {}
+
         /** Copy constructor, optional CopyOp object can be used to control
           * shallow vs deep copying of dynamic data.*/
         Object(const Object&,const CopyOp& copyop=CopyOp::SHALLOW_COPY);