correctly setup everything in the default run_once runner

[mockersf]

Objective

• Fix without_winit example panics #8658
• without_winit example panics thread 'Compute Task Pool (2)' panicked at 'called Option::unwrap()on aNone value', crates/bevy_render/src/pipelined_rendering.rs:134:84

Solution

• In the default runner method run_once, correctly finish the initialisation of the plugins. run_once can't be called twice so it's ok to do it there

[alice-i-cecile]

Can you add simple doc strings to run_once, and note the limitation of "should only be called once" there?

[mockersf]

I could, but it's not a new limitation that run_once can't be called twice. It's also not public.

To run it twice, one would have to do something like

    let mut app = App::new();
    app.add_plugins(DefaultPlugins.build().disable::<WinitPlugin>());
    app.run();
    app.run();

which doesn't work anyway because the run() method replaces the app with an empty one.

It feels a bit silly to put a comment "don't run this twice" on something called run_once to me.

[alice-i-cecile]

I still think that's useful context for future engine devs, but I won't block on it :)

[hymm]

We should probably add some documentation somewhere that the runner is responsible for calling these functions, but that should probably be a separate pr.