title: Round borders aren't clipping/masking perfectly
body: <p>I make a label in Interface Builder, with constraints for fixed height and fixed width:<br>
  <a href="https://i.stack.imgur.com/d8SXo.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/d8SXo.png" alt="enter image description here"></a></p>
  
  <p>I subclass it to give it a white round border:</p>
  
  <pre><code>class CircularLabel: UILabel {
      override func awakeFromNib() {
          super.awakeFromNib()
          layer.cornerRadius = frame.size.height / 2
          layer.borderColor = UIColor.white.cgColor
          layer.borderWidth = 5
          layer.masksToBounds = true
          clipsToBounds = true
      }
  }
  </code></pre>
  
  <p>But the clipping/masking isn't good at runtime:<br>
  <a href="https://i.stack.imgur.com/QKFC6.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/QKFC6.png" alt="enter image description here"></a></p>
  
  <p>I was expecting a perfect white border, without orange pixels.</p>
  
  <p>iPhone 8 (Simulator and real device), iOS 11.2, Xcode 9.2, Swift 3.2</p>
  
  <p><strong>MCVE at <a href="https://github.com/Coeur/stackoverflow48658502" rel="nofollow noreferrer">https://github.com/Coeur/stackoverflow48658502</a></strong></p>