Add Background Blur API

[Schweinepriester]

• Spec: https://w3c.github.io/mediacapture-extensions/#exposing-mediastreamtrack-source-background-blur-support
• Explainer: https://github.com/riju/backgroundBlur/blob/main/explainer.md
• Chrome:
  • https://chromestatus.com/feature/5077577782263808
  • Intent to Prototype
    • https://twitter.com/intenttoship/status/1539727761501192193
    • https://groups.google.com/a/chromium.org/g/blink-dev/c/3ke9hgaREwM/m/vthVWLwMAQAJ

Interoperability and Compatibility

Gecko: No signal, but spec LGTM ed by Jan-Ivar (Mozilla).
mozilla/standards-positions#658

WebKit: No signal but Apple co-author of explainer / PR.
https://lists.webkit.org/pipermail/webkit-dev/2022-June/032311.html

Web developers: No signals

Other signals: Almost every video-conference app has background blur feature.