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.

examples/osgparticle/osgparticle.cpp

@@ -385,7 +385,7 @@ int main(int argc, char **argv)
     viewer.setSceneData(root);
         
     // create the windows and run the threads.
-    //    viewer.realize(Producer::CameraGroup::ThreadPerCamera);
+    //    viewer.realize();
     // run single threaded since osgParticle still writes during cull.
     viewer.realize(Producer::CameraGroup::SingleThreaded);