Given a vector of widths (ws) and heights (hs) around a center
        (x_ctr, y_ctr), output a set of anchors (windows).

def _mkanchors(ws, hs, x_ctr, y_ctr):
        """
        Given a vector of widths (ws) and heights (hs) around a center
        (x_ctr, y_ctr), output a set of anchors (windows).
        """
        ws = ws[:, np.newaxis]
        hs = hs[:, np.newaxis]
        anchors = np.hstack((x_ctr - 0.5 * (ws - 1),
                             y_ctr - 0.5 * (hs - 1),
                             x_ctr + 0.5 * (ws - 1),
                             y_ctr + 0.5 * (hs - 1)))
        return anchors