title: Understanding animateKeyframes relative start times/delays
body: <p>I'm either being stupid, or misunderstanding how keyframe animations work on iOS (or both!). The two animation blocks below produce different results but I would expect them to be the same:</p>
  
  <pre><code>let duration: TimeInterval = 2
  
  UIView.animateKeyframes(withDuration: duration, delay: 0, animations: {
      UIView.addKeyframe(withRelativeStartTime: 0.9, relativeDuration: 0.1, animations: {
          self.someView.transform = CGAffineTransform(translationX: 0, y: 150)
      })
  })
  
  UIView.animateKeyframes(withDuration: duration * 0.1, delay: duration * 0.9, animations: {
      UIView.addKeyframe(withRelativeStartTime: 0, relativeDuration: 1, animations: {
          self.someView.transform = CGAffineTransform(translationX: 0, y: 150)
      })
  })
  </code></pre>
  
  <p>Can anyone help me understand why these are different when executed? The first seems to do what I expect, but the second one seems to execute the animation earlier than expected. </p>