Function begin_widget 

pub fn begin_widget(
    scene: &mut Scene,
    widget: &(impl Widget + ?Sized),
    index: Option<usize>,
    transform: Affine,
) -> Begun

Begin widget’s draws: index is its place among its parent’s children, when the caller knows it (a container that skips children it does not draw must say it, or the children after them would take their keys). transform is the one it draws with. Writes nothing unless widgets are keyed.