Function track 

pub fn track(
    plan: &hb_ot_shape_plan_t,
    face: &hb_font_t<'_>,
    scale: Scale,
    point_size: Option<f32>,
    buffer: &mut hb_buffer_t,
)

HB: hb_aat_layout_track

See https://github.com/harfbuzz/harfbuzz/blob/2c22a65f0cb99544c36580b9703a43b5dc97a9e1/src/hb-aat-layout.cc#L398