Function interpolate_deltas 

fn interpolate_deltas<C, D>(
    points: &[Point<C>],
    flags: &[PointFlags],
    contours: &[u16],
    out_points: &mut [Point<D>],
) -> Option<()>
where
    C: PointCoord,
    D: PointCoord + From<C>,

Interpolates the points that the current tuple did not reference, in the manner of the IUP hinting instruction.

Points carrying an explicit delta are marked with PointMarker::HAS_DELTA in flags. Each contour is handled independently.

Modeled after the FreeType implementation: https://github.com/freetype/freetype/blob/bbfcd79eacb4985d4b68783565f4b494aa64516b/src/truetype/ttgxvar.c#L3881