title: updateGL() with QTimer
body: <p>I have a problem with updating the OpenGL model on the screen. I use the timer to update the rotation angle every timer step:</p>
  
  <pre><code>QTimer *timer = new QTimer(this);
  connect(timer, SIGNAL(timeout()), this, SLOT(Spin()));
  timer-&gt;start(500);
  
  void GLWidget::Spin()
  {
      qDebug() &lt;&lt; "Spin()";
  
      rotationX  += 5;
      rotationY  += 5;
      rotationZ  += 5;
      updateGL();
  
  }
  </code></pre>
  
  <p>I also tried update() instead of updateGL() but it does not work either. 
  The weird thing is, by rotating the model with the mouse it does work:</p>
  
  <pre><code>void GLWidget::mouseMoveEvent(QMouseEvent *event)
  {
      GLfloat dx = GLfloat(event-&gt;x() - lastPos.x()) / width();
      GLfloat dy = GLfloat(event-&gt;y() - lastPos.y()) / height();
      if (event-&gt;buttons() &amp; Qt::LeftButton) {
          rotationX += 180 * dy;
          rotationY += 180 * dx;
          updateGL();
      } else if (event-&gt;buttons() &amp; Qt::RightButton) {
      rotationX += 180 * dy;
          rotationZ += 180 * dx;
          updateGL();
      }
      lastPos = event-&gt;pos();
  }
  </code></pre>