Write image data.

        Parameters
        ----------
        img : `PIL.Image.Image`
            Image.
        data : `iterable` of `int`
            Image data.
        tr : `markovchain.image.traversal.Traversal`
            Image traversal.
        x : `int`
            X offset.
        y : `int`
            Y offset.

def _write_imgdata(img, data, tr, x=0, y=0):
        """Write image data.

        Parameters
        ----------
        img : `PIL.Image.Image`
            Image.
        data : `iterable` of `int`
            Image data.
        tr : `markovchain.image.traversal.Traversal`
            Image traversal.
        x : `int`
            X offset.
        y : `int`
            Y offset.
        """
        for pixel, (x1, y1) in zip(data, tr):
            img.putpixel((x + x1, y + y1), pixel)
        return img