Use setAlpha to cancel the fade out animation and avoid race condition.

The race condition this fixes is as follows:

// _visible = NO, _imageView = nil
setVisible(YES)
// _visible = YES, _imageView = UIImageView
setVisible(NO)
// _visible = NO, _imageView = UIImageView, animation running
setVisible(YES)
// _visible = YES, _imageView = UIImageView, animation running
setVisible(NO)
// _visible = NO, _imageView = UIImageView, 2x animations running
// animation 1 completed
// animation 2 completed - crash