Function fill_role 

pub fn fill_role(
    scene: &mut Scene,
    transform: Affine,
    shape: &RoundedRect,
    role: SurfaceRole,
    theme: &Theme,
    ground: AlphaColor<Srgb>,
)

Fill shape as a surface of role stands: its finish’s painter set (Theme::painter) draws it over ground. A floating or bar surface under an active BackdropRecording can show what lies below it, held in the material’s band, bent and lit as its set says; every other surface, and any without a backdrop pass, is drawn opaque.