title: CABasicAnimation with CALayer.affineTransform not working
body: <p>I have below code specifed as a orbit and planet demo:</p>
  
  <pre><code>    _orbitLayer.bounds = CGRectMake(0, 0, radius*2, radius*2);
      _orbitLayer.position = center;
      _orbitLayer.cornerRadius = radius;
      _orbitLayer.borderColor = [UIColor redColor].CGColor;
      _orbitLayer.borderWidth = 1.5;
      if ([_orbitLayer superlayer]) {
          [_orbitLayer removeAllAnimations];
          [_orbitLayer removeFromSuperlayer];
      }
      [self.layer insertSublayer:_orbitLayer above:_stripeLayer];
  
      // planet circyle
      _dotCircyleLayer.bounds = CGRectMake(0, 0, (arcWidth - 10),(arcWidth - 10));
      CGPoint anchor_center = CGPointMake(_orbitLayer.bounds.size.width,
                                          _orbitLayer.bounds.size.height/2);
      _dotCircyleLayer.position = anchor_center;
      _dotCircyleLayer.cornerRadius = (arcWidth - 10)/2;
      _dotCircyleLayer.backgroundColor = anchorColor.CGColor;
      if ([_dotCircyleLayer superlayer])
          [_dotCircyleLayer removeFromSuperlayer];
      [_orbitLayer addSublayer:_dotCircyleLayer];
      _orbitLayer.affineTransform = CGAffineTransformMakeRotation(startAngle);
  
  // animation
      CABasicAnimation *arcAnimation = [CABasicAnimation animationWithKeyPath:@"affineTransform"];
      arcAnimation.duration = 0.8;
      arcAnimation.fromValue = [NSValue valueWithCGAffineTransform:CGAffineTransformMakeRotation(startAngle)];
      arcAnimation.toValue = [NSValue valueWithCGAffineTransform:CGAffineTransformMakeRotation(endAngle)];
      arcAnimation.removedOnCompletion = NO;
      [_orbitLayer addAnimation:arcAnimation forKey:nil];
  </code></pre>
  
  <p>I could rotate this orbit to simulate planet circyling by <code>_orbitLayer.affineTransform = CGAffineTransformMakeRotation(startAngle);</code>
  But I cannot animate <code>_orbitLayer.affineTransform</code>. My animation code does not work. What's wrong with it?</p>