Function modal 

pub fn modal(
    scene: &mut Scene,
    transform: Affine,
    rect: Rect,
    openness: f32,
    theme: &Theme,
)

Dim rect (the whole window) behind a modal Panel. openness runs from 0 (closed) to 1 (open), so the scrim fades in with the panel. Call after everything it dims is drawn into the same scene.