title: Get rectangle from GPUImageHarrisCornerDetectionFilter or GPUImageHoughTransformLineDetector
body: <p>I am working on Business Card or Licence Card scanning in iOS. Before i use OpenCV but openCV is too slow for realtime card detection via camera. I switched to GPUImage open source library. The problem is how i can get rectangle from the four corner points using <code>GPUImageHarrisCornerDetectionFilter</code> or a rectangle area formed by lines crossing each other using <code>GPUImageHoughTransformLineDetector</code></p>