Content suggestion: ImageDecoder & ImageTrackList how-to/tutorial

## What is the new suggestion?
Webcodecs introduce ImageDecoder and ImageDecoder come with ImageTrackList.
It sounds like a nice feature but I can't figure out how to provide an image file containing multiple tracks...
I even never heard about tracks inside an image and google doesn't help me at all with that.

ImageTrackList exists so I'm sure it's doable but how to package multiple tracks inside a single image file ?

## Why is it important or useful?
I think it would be a great way to organised assets compared to a spritesheet but I have no idea how to create the multi-tracks-image. What tool am I suppose to use ? What would be the format of the file ?

"Does it address critical needs in the web industry?"
Probably not but this feature existed in Adobe Flash (the ability to package multiple assets, easy to target, inside a single file) and I really miss it. The idea I could do it again soon with ImageDecoder & ImageTrackList makes me very happy. Please show us a step by step example to create such images .

MDN URL: https://developer.mozilla.org/en-US/docs/Learn/
MDN URL: https://developer.mozilla.org/en-US/docs/Web/API/TextEncoder