Function build_scene_at_time 

pub fn build_scene_at_time<'a>(
    children: &'a [ChildComponent],
    viewport: (f32, f32),
    root_css: CssStyle,
    anim: BuildAnimationCtx,
) -> BuiltScene<'a>

Like build_scene_with_root but resolves animations at time (seconds) for each node and merges the result into its CssStyle. Use this when rendering an animated frame.