Function im_draw_line_user 

pub unsafe extern "C" fn im_draw_line_user(
    image: *mut VipsImage,
    x1: c_int,
    y1: c_int,
    x2: c_int,
    y2: c_int,
    plot: VipsPlotFn,
    a: *mut c_void,
    b: *mut c_void,
    c: *mut c_void,
) -> c_int