Background Blur: Unprocessed video should be mandatory to support

[alvestrand]

In the context of https://w3c.github.io/mediacapture-extensions/#exposing-mediastreamtrack-source-background-blur-support and similar mechanisms for platform effects on video tracks, concern has been raised that this type of functionality can cause deep confusion for users when the same class of effect can be applied both in an application (conferencing systems, for instance) and in the platform - users won't know where to turn it off, and double blur may cause effects that were not as intended.

For other types of effects (face touchups, background replacement, emoji reactions), the cost of being unable to prevent confusing interactions between effects is even bigger.

This concern would be alleviated if all such capabilities were defined to be "MUST support constraint=false" - that is, if the application could always turn them off and get an unprocessed video feed.