Allow passing loader instances to `useLoader`

[mattrossman]

Allow passing loader instances to `useLoader`

[CodyJasonBennett]

Does #3131 satisfy this?

[mattrossman]

@CodyJasonBennett That looks almost exactly like what I want!

The only difference I see is the caching behavior. For example, in this case:

useLoader.preload(gltfLoader, [URL_A, URL_B])

function ComponentA() {
  const gltf = useLoader(gltfLoader, URL_A)
}

In the current code, the suspend function sees these as different cache keys ([loader, URL_A] instead of [loader, URL_A, URL_B]) and would re-run the Loader.load function instead of returning the preloaded object.

This sandbox shows one way to address this, memoizing load results for each URL per-loader instead of just memoizing loader instances.

Something like like:

                    // memoizedResults[<loader>][<url>] = <result> | undefined
const memoizedResults = new WeakMap<Loader<unknown>, Map<string, unknown>>()

That way even though suspend re-runs, it skips loading a new object.