Add pdf support

[johnshaughnessy]

Add support for loading pdfs with bitecs.

Also rework coroutines by introducing JobRunner.

[johnshaughnessy]

It would be nice to avoid this call to anyEntityWith.

One option is to memoize menu, as in

let menu;
export function pdfMenuSystem(world: HubsWorld, sceneIsFrozen: boolean) {
  menu = menu || anyEntityWith(world, PDFMenu)!;
  ...

Another option that seems nice is to set menu in a preload, since it feels correct to assign it as part of initialization. This commit explores that idea bbbe398 . The calling code looks like this:

let menu: EntityID;
preload(
  repeatUntilTrue(() => {
    menu = (window.APP && APP.world && anyEntityWith(APP.world, PDFMenu)) as EntityID;
    return !!menu;
  })
);

The inner function will be repeated in a setInterval until it returns true.

I don't know the best way to handle cases like this where we are assuming that some long-lived entity will be created during initialization, and we want to hold a reference to it. I'd like to avoid sticking more things on APP when this comes up.

For now, the simplest solution I could come up with is to just call anyEntityWith every tick (with a ! for type coercion), even though it'll be the same entity every time.

[netpro2k]

For video menu we just made a query and grabbed the first element, which is basically the same as anyEntityWith.

I definitely like the idea of doing it in a preload. The setInterval seems surprising. Ideally we should be able to do it synchronously. Maybe the functions you pass to prelaod get called after APP has been set up and passed a world. Some obviously will also be async after that, but this one shouldn't need to be.

We could also just have a way to get the world with a promise which would remove the need to modify preload.