Added support for optionally calling releaseContext at the end of each

renderinTraversals() to help with cases where uses are driving multiple
contexts from mulitple viewers in a single threaded frame loop.

src/osgViewer/CompositeViewer.cpp

@@ -184,6 +184,8 @@ int CompositeViewer::run()
         }
     }
         
+    setReleaseContextAtEndOfFrameHint(false);
+
     return ViewerBase::run();
 }