pub unsafe extern "C" fn gst_buffer_add_audio_downmix_meta(
    buffer: *mut GstBuffer,
    from_position: *const GstAudioChannelPosition,
    from_channels: c_int,
    to_position: *const GstAudioChannelPosition,
    to_channels: c_int,
    matrix: *mut *const c_float
) -> *mut GstAudioDownmixMeta