Add a white tile border to indicate the swap.

def _draw_swap_cv(self, board_image, swap):
        """Add a white tile border to indicate the swap."""
        tile_h, tile_w = self._TILE_SHAPE[0:2]
        # get a single bounding box
        (row_1, col_1), (row_2, col_2) = swap
        t = tile_h * min(row_1, row_2)
        b = tile_h * (1 + max(row_1, row_2))
        l = tile_w * min(col_1, col_2)
        r = tile_w * (1 + max(col_1, col_2))
        top_left = (l, t)
        bottom_right = (r, b)
        data.cv2.rectangle(board_image, top_left, bottom_right,
                           color=(255, 255, 255), thickness = 4)