Function immediate_mode_subprogram

An 'immediate mode' subprogram is one that runs in two phases: it updates some state in one phase and updates that state based on messages in another. This is a style used in libraries like egui, but is adapted here for flo_scene to support a wide range of possible interactions. This takes two functions. `update_state()` takes the previous state along with a future that receives the input for th

(
    input:          InputStream<TInput>, 
    context:        SceneContext, 
    update_state:   impl 'static + Send + Sync + for<'a> Fn(BoxFuture<'a, Option<Vec<TInput>>>, TState, SceneContext) ->