[webrtc-encoded-transform] expose rid (and mid) as metadata on outgoing frames (#147)

fippo has just created a new issue for https://github.com/w3c/webrtc-encoded-transform:

== expose rid (and mid) as metadata on outgoing frames ==
it seems that we're not exposing `rid` as metadata on the outgoing stream (on incoming rid should be undefined; mid might be marginally useful):
  https://w3c.github.io/webrtc-encoded-transform/#scriptTransform
Given the general thinking (that I don't agree with but that is a different story) that mid/rid is cooler than synchronizationSource should we expose it? Does the same apply to mid?

Please view or discuss this issue at https://github.com/w3c/webrtc-encoded-transform/issues/147 using your GitHub account


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