title: Implement ink annotations on iOS 11 PDFKit document
body: <p>I want to allow the user to draw on an iOS 11 PDFKit document viewed in a PDFView. The drawing should ultimately be embedded inside the PDF.</p>
  
  <p>The latter I have solved by adding a PDFAnnotation of type "ink" to the PDFPage with a UIBezierPath corresponding to the user's drawing.</p>
  
  <p>However, how do I actually record the touches the user makes on top of the PDFView to create such an UIBezierPath?</p>
  
  <p>I have tried overriding touchesBegan on the PDFView and on the PDFPage, but it is never called. I have tried adding a UIGestureRecognizer, but didn't accomplish anything.</p>
  
  <p>I'm assuming that I need to afterwards use the PDFView instance method convert(_ point: CGPoint, to page: PDFPage) to convert the coordinates obtained to PDF coordinates suitable for the annotation.</p>