Changed the Viewer::realize() calls across to not using the threading paramter

leaving it up to the Viewer to specify the mode (which by default is MultiThreaded).
Added a check for the presence of osgParticle systems so that threading is
disabled in this case.

include/osgProducer/OsgCameraGroup

@@ -137,9 +137,11 @@ class OSGPRODUCER_EXPORT OsgCameraGroup : public Producer::CameraGroup
 
         void advance();
 
+        /** Set the threading model and then call realize().*/
+        virtual bool realize(ThreadingModel thread_model );
 
         /** Realize the render surfaces (OpenGL graphics) and various threads, and call any realize callbacks.*/
-        virtual void realize( ThreadingModel thread_model= SingleThreaded );
+        virtual bool realize();
 
         /** Set the model view matrix of the camera group,
           * by individually set all the camera groups's camera.*/