title: QGraphicsItem position after changing boundingRect
body: <p>I have class derived from <code>QGraphicsItem</code>. It contains vector of points wich I draw in paint:</p>
  
  <pre><code>for(int i = 0; i &lt; _vertexes.size(); i++)
  {
       ...
    painter-&gt;drawEllipse(_vertexes[i], POINT_RADIUS, POINT_RADIUS);
  }
  </code></pre>
  
  <p>when I add point in <code>_vertexes</code> with this code</p>
  
  <pre><code> prepareGeometryChange();
   _vertexes.pop_back();
  </code></pre>
  
  <p>position of points in the view is changing, <code>boundingRect</code> is calculated using <code>_vertexes</code></p>
  
  <p>How to save points positions? I don't want all points change position after adding new one if new <code>boundingRect</code> is bigger. By the <code>pos()</code> returns always the same position (0, 0) but it could be in a different position of screen.</p>