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/osgUtil/RenderLeaf

@@ -34,6 +34,7 @@ class OSGUTIL_EXPORT RenderLeaf : public osg::Referenced
     
 
         inline RenderLeaf(osg::Drawable* drawable,osg::RefMatrix* projection,osg::RefMatrix* modelview, float depth=0.0f):
+            osg::Referenced(false),
             _parent(0),
             _drawable(drawable),
             _projection(projection),
@@ -76,6 +77,7 @@ class OSGUTIL_EXPORT RenderLeaf : public osg::Referenced
 
         /// disallow creation of blank RenderLeaf as this isn't useful.
         RenderLeaf():
+            osg::Referenced(false),
             _parent(0),
             _drawable(0),
             _projection(0),
@@ -83,7 +85,7 @@ class OSGUTIL_EXPORT RenderLeaf : public osg::Referenced
             _depth(0.0f) {}
 
         /// disallow copy construction.
-        RenderLeaf(const RenderLeaf&):osg::Referenced() {}
+        RenderLeaf(const RenderLeaf&):osg::Referenced(false) {}
         /// disallow copy operator.
         RenderLeaf& operator = (const RenderLeaf&) { return *this; }
 

include/osgUtil/StateGraph

@@ -60,6 +60,7 @@ class OSGUTIL_EXPORT StateGraph : public osg::Referenced
 
 
         StateGraph():
+            osg::Referenced(false),
             _parent(NULL), 
             _stateset(NULL), 
             _depth(0),
@@ -70,6 +71,7 @@ class OSGUTIL_EXPORT StateGraph : public osg::Referenced
         }
 
         StateGraph(StateGraph* parent,const osg::StateSet* stateset):
+            osg::Referenced(false),
             _parent(parent), 
             _stateset(stateset),
             _depth(0),