Build a plane.position from CD matrix

    points = []
    vertices = []
    segment_type = SegmentType['MOVE']

    for x, y in ([0, 0], [1, 0], [1, 1], [0, 1 ]):
        V = w.all_pix2world(x*header['naxis1'], y*header['naxis2'], 1, 1, 1)
        points.append(Point(float(V[0]), float(V[1])))
        vertices.append(Vertex(float(V[0]), float(V[1]), segment_type))
        segment_type = SegmentType['LINE']
    vertices.append(Vertex(points[0].cval1,
                           points[0].cval2,
                           SegmentType['CLOSE']))

    polygon = shape.Polygon(points=points, samples=shape.MultiPolygon(vertices))
    position = Position(time_dependent=False,
                        bounds=polygon)
    plane.position = position