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/osgvertexprogram/osgvertexprogram.cpp

@@ -402,7 +402,7 @@ int main(int argc, char *argv[])
     viewer.setSceneData(rootnode);
     
     // create the windows and run the threads.
-    viewer.realize(Producer::CameraGroup::ThreadPerCamera);
+    viewer.realize();
 
     while( !viewer.done() )
     {