[webrtc-pc] simulcast offer doesn't work with addTrack. An omission (#2147)

jan-ivar has just created a new issue for https://github.com/w3c/webrtc-pc:

== simulcast offer doesn't work with addTrack. An omission ==
https://github.com/w3c/webrtc-pc/pull/2081 put all simulcast processing of offers from SFUs under:

 *"3. If no suitable transceiver is found (transceiver is unset), run the following steps:"*

This means that when a suitable transceiver from *addTrack* **is** found—[section 5.10](https://tools.ietf.org/html/draft-ietf-rtcweb-jsep-25#section-5.10) says they're suitable—then we succeed without processing the simulcast rids, a mistake.

To solve this, either we modify JSEP to say it's unsuitable, or add language to set the existing transceiver's [[SendEncodings]].

I see no reason why we can't do the latter. We've seen real-world interest in having *addTrack* work with simulcast.

We may need to bump the [transactionId](http://w3c.github.io/webrtc-pc/#dom-rtcrtpsendparameters-transactionid), but that's a whole other issue.

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/2147 using your GitHub account