downloadBmpImageTsplCommandData method
转成打印机可识别的tspl命令数据
`pos` 打印的x,y位置
Implementation
Future<Uint8List> downloadBmpImageTsplCommandData(
        {required (int, int) pos}) =>
    RustLib.instance.api
        .crateApiImageBitmapImageDownloadBmpImageTsplCommandData(
            that: this, pos: pos);