Function paint_surface 

pub fn paint_surface(
    scene: &mut Scene,
    transform: Affine,
    rect: Rect,
    level: SurfaceLevel,
    theme: &Theme,
) -> Theme

Paint level over rect: its shadow, its fill and its edge. Returns the theme a child standing on it paints with (Theme::on_ground): ink, lifts and the raised well re-measured against the level’s fill.