title: Crop uiimage as screen size
body: <p>My application have an function that allow user to move and scale the image just like setting allowsEditing = YES with UIImagePickerController.</p>
  
  <p>My approach is having a scrollview which allow user to zoom with level 1-4
  There is only a image view on the scrollview.</p>
  
  <p>At the end, I want to crop the image as the screen size. How can I do it?</p>