UIImage as CAShapeLayer contents
===

If I create a CAShapeLayer with just a background color it shows up, but if i set the content to an image, nothing shows. 
 

    CAShapeLayer* leftDot = [CAShapeLayer layer];
    leftDot.opacity = 1.0;
    leftDot.frame = CGRectMake(leftRef.CGPointValue.x, leftRef.CGPointValue.y, 2 * radius, 2 * radius);
    leftDot.position = CGPointMake(leftRef.CGPointValue.x + 4, leftRef.CGPointValue.y + 4);
    leftDot.cornerRadius = radius;
    [leftDot setContents:[UIImage imageNamed: @"glow.png"]];

    [self.layer addSublayer: leftDot];
