Function paint_selector 

pub fn paint_selector(
    scene: &mut Scene,
    transform: Affine,
    shape: &RoundedRect,
    role: SelectorRole<'_>,
    theme: &Theme,
)

Paint one selector: a strip body or an item, on shape.

An item paints its fill, its edge and its lit rim, and nothing else: no shadow, since a lift means “chosen on the surface” and a shadow “off it” (HIG §9.3). The rim is one pixel inside the edge and fades out rim_span of the way down, so it reads as light catching the top of the item rather than a ring around it.