void cut(struct Png *image, int x1, int y1, int x2, int y2) {
    int y, x;
    for (y = 0; y <= y2 - y1; y++) {
        png_byte *cut_row1 = image->row_pointers[y + y1];
        for (x = 0; x <= x2 - x1; x++) {
            png_byte *row = image->row_pointers[y];
            png_byte *ptr = &(row[x * 3]);
            png_byte *cut_ptr1 = &(cut_row1[(x + x1) * 3]);
            ptr[0] = cut_ptr1[0];
            ptr[1] = cut_ptr1[1];
            ptr[2] = cut_ptr1[2];
        }
    }
    for (y = y2 - y1 + 1; y < image->height; y++)
        free(image->row_pointers[y]);
    image->height = y2 - y1;
    image->width = x2 - x1;
}