title: UIView.animateKeyFramesWithDuration not animating smoothly
body: <p>I am trying to write a blinking animation, but the timing of it feels off. I made a simple version in a playground:</p>
  
  <pre><code>import UIKit
  import XCPlayground
  let v = UIView()
  v.frame.size = CGSize(width: 200, height: 200)
  v.backgroundColor = UIColor.redColor()
  v.layer.cornerRadius = 100
  
  UIView.animateKeyframesWithDuration(1, delay: 0, options: .Repeat, animations: {
      UIView.addKeyframeWithRelativeStartTime(0, relativeDuration: 0.5, animations: {
        v.alpha = 0.0
      })
      UIView.addKeyframeWithRelativeStartTime(0.5, relativeDuration: 0.5, animations: {
        v.alpha = 0.5
      })
      }, completion: nil)
  
  XCPlaygroundPage.currentPage.liveView = v
  </code></pre>
  
  <p>The view fades from 0 to 0.5 and then appears to stutter at full alpha for a second before resuming the animation. I noticed the same thing in the simulator. </p>
  
  <p>Is there something I am missing about how the keyframes are supposed to work?</p>