bitECS: Support media with link component

[keianhzo]

Fixes #6263

In the AFrame loader when an object had a link and a image components (also works with video and loader) the media would override the link media

In the new loader we don't support a media loader loading multiple media. The second media would override the first one's parameters.

Using image/video/model with the link component seems to be a pretty common use case. This PRs adds a hack to support that specific use case.

Another alternative would be to add support for multiple loads to the media loader but I'm no sure if that's something that we want...

[takahirox]

As commented inline this solution is very hacky for sure. Give me some time to think whether there is clearer solution...

I haven't deeply thought yet but making a new component (that holds link url) for this special case rather than adding link url to MediaLoader component would be simpler? Anyways give me more time...

[takahirox]

Let me share my thoughts.

With the new loader enabled, we prevent a glTF node that has multiple Hubs components that generates Three.js object. We preprocess such nodes to make multiple Hubs Component glTF nodes under the node. Under this policy, Link + Media is not doable as is.

Probably we don't spend long time for this problem. So probably preprocessing Link + Media glTF Hubs Components and reusing the existing code as much as possible would be good. Also, adding a special path after loading a media as this PR does would be reasonable.

But I don't prefer adding linkSrc property to MediaLoader because MediaLoader component will have strong dependency with Link. Maybe introducing a new component (eg: MediaLink) would be simpler.

This code in the media loading system

  const linkSrc = APP.getString(MediaLoader.linkSrc[eid]);
  if (linkSrc) {
    ...
  }

would be replaced with like

  if (hasComponent(world, MediaLink, eid)) {
    const linkSrc = APP.getString(MediaLink.src[eid]);
    ...
  }

[keianhzo]

Yes, that makes sense and I like that is much more clear, updated.