Function paint_rule 

pub fn paint_rule(
    scene: &mut Scene,
    transform: Affine,
    theme: &Theme,
    rect: Rect,
    edge: RuleEdge,
    surface: Option<AlphaColor<Srgb>>,
)

Paint the rule a Divider draws along edge of rect, inside it, for a surface painted by hand. surface is the ground it stands on when that is not surfaces.base (as Divider::on).