Function separator 

pub fn separator(
    scene: &mut Scene,
    transform: Affine,
    x0: f64,
    x1: f64,
    y: f64,
    inset: f64,
    surface: AlphaColor<Srgb>,
)

A 1 px separator between rows inside a surface: derived against surface, inset inset from both ends (HIG §9.5: spacing.s3, never full-bleed into a corner radius), centred on y.