title: Change radius to MKCircle on MKMapView with UISlider
body: <p>I'm trying to change the radius of <code>MKCircle</code> by moving an <code>UISlider</code>.
  The problem is that the <code>radius</code> property of <code>MKCircle</code> is <em>readonly</em>.</p>
  
  <p>I tried in this way:</p>
  
  <pre><code>- (void)sliderValueChanged:(UISlider *)sender
  {
      self.radiusLabel.text = [NSString stringWithFormat:@"%i mt", value];
  
      MKCircle * oldC = (MKCircle *)self.mapView.overlays[0];
  
      MKCircle * c = [MKCircle circleWithCenterCoordinate:self.lastLocation.coordinate radius:value];
  
      [self.mapView addOverlay:c];
  
      [self.mapView removeOverlay:oldC];
  }
  </code></pre>
  
  <p>but the render is very slow and the circle is rendered tile-by-tile (like a pdf in quartz).
  Is there a better and faster way to change the radius ?</p>
  
  <p>Thanks...</p>