Detach Offscreen Canvas task to a web worker

[Mazuh]

To ease the main thread and not interfere with the rest of the user experience, it'd be cool to have any heavy frame-by-frame processing in a web worker.

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However, this task is blocked.

I couldn't see a way to make it possible without this other issue being solved first: google-ai-edge/mediapipe#3659
(And I tried a lot!)

When unblocked, see this for reference:

• https://github.com/webrtc/samples/tree/gh-pages/src/content/insertable-streams/video-crop/js
• https://codesandbox.io/s/yv926onnjv?from-embed=&file=/src/index.ts