title: didChangeCameraPosition get bounding box/ rectangle of available coordinates
body: <p>How can I retrieve the GMSCoordinateBounds from a GMSCameraPosition? I want to know the visible coordinates on the map (at least Northeast/Southwest points) everytime the user moves the camera as in:</p>
  
  <pre><code>(void)mapView:(GMSMapView *)mapView didChangeCameraPosition:(GMSCameraPosition *)position
  </code></pre>