Change video-processing sample to process on a worker

[dogben]

Following up to w3c/mediacapture-transform#28, the sample at https://github.com/webrtc/samples/tree/gh-pages/src/content/insertable-streams/video-processing should be changed to (optionally?) process the video on a worker rather than the main thread.

[fippo]

@dogben do you still want to do this? The cropping sample uses a worker so we have an example of how it should be done

[dogben]

I think it would be a good idea, since the final spec that reached consensus (https://www.w3.org/TR/mediacapture-transform/) has the MediaStreamTrackProcessor and VideoTrackGenerator exposed only on DedicatedWorker. Making the sample fully spec-compliant seems desirable.

@alvestrand