Function im_fastlineuser 

pub unsafe extern "C" fn im_fastlineuser(
    im: *mut VipsImage,
    x1: c_int,
    y1: c_int,
    x2: c_int,
    y2: c_int,
    fn_: VipsPlotFn,
    client1: *mut c_void,
    client2: *mut c_void,
    client3: *mut c_void,
) -> c_int