fix opencv-4.6.* dnn inference

        im_h, im_w = img.shape[:2]

        blks, mask, lines_map = self.net(img_in)

        if self.backend == 'opencv':

            if mask.shape[1] == 2:     # some version of opencv spit out reversed result

                tmp = mask

                mask = lines_map

                lines_map = tmp

        resize_ratio = (im_w / (detect_size[0] - dw), im_h / (detect_size[1] - dh))

        blks = postprocess_yolo(blks, self.conf_thresh, self.nms_thresh, resize_ratio)