title: QGraphicsView level of detail handling
body: <p>I am working on a visualization SW which uses QGraphicsView and QGraphicsScene (Qt4.8, PyQt). I need LOD (level of detail) handling depending on zoom level. For this I create more QGraphicsScenes and I render the whole scene to each with different LOD level. (There is a QGraphicsScene for each LOD level).
  I switch between these QGraphicsScenes depending on the zoom level. (QGraphicsView.setScene()).
  The big problem is that the QGraphicsView's ScrollBars resets their positions when setScene() is called.</p>
  
  <p>Here is a code snippet which tries to restore the ScrollBars values, but it doesn't work:</p>
  
  <pre><code>        hsb = self.horizontalScrollBar()
          vsb = self.verticalScrollBar()
          hv, vv = hsb.value(), vsb.value()
          self.lod = i
          self.setScene(self.scenes[self.lod])
          hsb.setValue(hv)    # this simply doesn't work !!!!
          vsb.setValue(vv)
  </code></pre>
  
  <p>Any idea to retain the positions of the ScrollBars? Or any idea for efficient LOD handling?
  I could try to connect the ScrollBar's value changed signals into some custom slots which would disconnect from its own signal than re-set the ScrollBar's value from a saved one. That would be a very lame and ugly solution.</p>