[webrtc-svc] Handling of unknown scalability modes is underspecified (#57)

alvestrand has just created a new issue for https://github.com/w3c/webrtc-svc:

== Handling of unknown scalability modes is underspecified ==
Consider the following snippet:
```
    const pc = new RTCPeerConnection();
    t.add_cleanup(() => pc.close());
    const { sender } = pc.addTransceiver('video', {
      sendEncodings: [{scalabilityMode: 'TotalNonsense'}],
    });
    const param = sender.getParameters();
    const encoding = getFirstEncoding(param);
```
What should the value of encoding.scalabilityMode be?

This matters a lot for the case where "supported scalability modes" is not part of the API.
(At the moment the Chrome implementation returns 'TotalNonsense')


Please view or discuss this issue at https://github.com/w3c/webrtc-svc/issues/57 using your GitHub account


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