title: Immediately freeze-frame an AVCaptureSession
body: <p>I'm building an app that allows users to snap pictures using the iPhone camera, and am using AVFoundation when it's available (iOS4) so users can use the tap-to-focus feature, even with a custom overlay.</p>
  
  <p>The problem I'm having is that <strong><em>captureStillImageAsynchronouslyFromConnection</em></strong> takes several seconds to complete, during which I'd like to freeze-frame the main video feed so there isn't a confusing state where the video is still running, but the user has snapped a photo (and the camera shutter sound has gone off). </p>
  
  <p>I've tried calling [session stopRunning] right after I request a still image capture, but this leads to unpredictable results; the still image completion block often doesn't get fired in that case.</p>
  
  <p>Any ideas on how I could have the video preview layer "pause" as soon as the shutter is hit?</p>