Description:
------------
When using addImage, then behavior depends on the number of frames already in 
the gmagick instance :

When using the following code :

class layers
{
    private $gmagick;
    public function __construct(Gmagick $gmagick)
    {
        $this->gmagick = $gmagick;
    }

    function set($offset, $frame)
    {
        $this->gmagick->setimageindex($offset);
        // tricky workaround for https://bugs.php.net/bug.php?id=62309
        $this->gmagick->nextimage();
        $this->gmagick->addimage($frame);
    }
}

If the gmagick object contains only one frame at the beginning, the frame is 
inserted BEFORE the existing frame, otherwise it is inserted after