title: How do I get tap locations (GestureDetector)?
body: <p>I'm trying to implement double-tap-to-zoom in my <a href="https://github.com/perlatus/flutter_zoomable_image/blob/3515144d1ce43d7363258ef51e3d4c7e4e1c22ab/lib/src/zoomable_image.dart#L120" rel="noreferrer">zoomable_images plugin</a> but the <a href="https://docs.flutter.io/flutter/gestures/GestureTapCallback.html" rel="noreferrer">GestureTapCallback</a> doesn't provide the tap location information.</p>
  
  <p>Ideally the offset would be returned by the callback. Is there another API for this?</p>