title: I can't understand SpriteKit's rotation system
body: <p>I'm trying to rotate sprites with SKAction.rotateToAngle. For example:  </p>
  
  <pre><code>rotate = SKAction.rotateToAngle(desiredAngle, duration: 1.0)
  sprite.runAction(rotate)
  </code></pre>
  
  <p>But it didn't work as expected. The sprites rotates in a different way. By trying and trying I've found out that this is the rotation system adopted by SpriteKit:  </p>
  
  <p><a href="https://i.stack.imgur.com/vsEKH.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/vsEKH.png" alt="enter image description here"></a></p>
  
  <p>Now I don't know why they decided to adopt this coordinate system but it's pretty counter-intuitive. How can I convert between this system and the normal one?
  And by the way how can I rotate the sprites clockwise?</p>