From Tamer Fahmy, Fix for bug when scrolling down and using render on demand.

"Currently issuing a mouse scroll DOWN event would stop updating
animations in progress.

The fix consists of changing the line
 us.requestContinuousUpdate( false );
to:
 us.requestContinuousUpdate( isAnimating() || _thrown );

in OrbitManipulator::handleMouseWheel() as has been done for the
GUIEventAdapter::SCROLL_UP case a couple of lines earlier or in
src/osgGA/FirstPersonManipulator.cpp."

src/osgGA/OrbitManipulator.cpp

@@ -261,7 +261,7 @@ bool OrbitManipulator::handleMouseWheel( const GUIEventAdapter& ea, GUIActionAda
             // perform zoom
             zoomModel( -_wheelZoomFactor, true );
             us.requestRedraw();
-            us.requestContinuousUpdate( false );
+            us.requestContinuousUpdate( isAnimating() || _thrown );
             return true;
         }