async'ize requestInit()

[j6k4m8]

Layer#requestInit should be async so that visualizer can .then it

requestInit() {
    return doAsyncPromiseThing();
}

[j6k4m8]

Visualizer can Promise.all() the requestInits, and then start the animation-loop on their successful returns

[j6k4m8]

Visualizer#triggerRender() should run automatically once all layers' requestInits resolve successfully. This means you should be able to attach your own promises to a Visualizer so that you can control the triggerRender call, if you want:

V = new Visualizer({
    targetElement: "...",
    renderLayers: { ... },
    beforeLoad: [
        (viz) => {
            return fetch("mywebsite.com/api").then(r => r.json()).then(data => {
                viz.customData = data;
            });
        }
    ]
}

...where beforeLoad (can settle on a different name for this) is an array of promises that can be concatenated to the requestInits.

[j6k4m8]

This possibly also means that we want an optional boolean flag autoRender or somesuch that can be set to false so that the onus is on the user to explicitly call triggerRender.