Setting default surface format

As suggested by the principal developer of OpenScenegGraph in the
following thread

  https://groups.google.com/forum/#!msg/osg-users/A89jTUGq4VY/RsmqvF1JAwAJ

I am now setting a default profile for the Qt application to force usage
of OpenGL 2.1. I am not sure whether this may solve issue #4, but let's
give it a try.

QtOSG.cpp

@@ -1,11 +1,18 @@
 #include "MainWindow.h"
 
 #include <QApplication>
+#include <QSurfaceFormat>
 
 int main( int argc, char** argv )
 {
   QApplication application( argc, argv );
 
+  QSurfaceFormat format;
+  format.setVersion(2, 1);
+  format.setProfile( QSurfaceFormat::CompatibilityProfile );
+
+  QSurfaceFormat::setDefaultFormat(format);
+
   MainWindow mainWindow;
   mainWindow.show();