'Return (xmin, xmax, ymin, ymax).'

@property
def extents(self):

    (x0, y0, width, height) = self._rect_bbox
    (xmin, xmax) = sorted([x0, (x0 + width)])
    (ymin, ymax) = sorted([y0, (y0 + height)])
    return (xmin, xmax, ymin, ymax)
