glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);// Puffer loeschen
   glLoadIdentity();
   glRotatef(20.0,1.0,0.0,0.0);
   glRotatef(30.0,0.0,1.0,0.0);
   Wuerfel(extent);
   glFlush();