Support for multi-stream WebRTC calls (SPEC-252)

[matrixbot]

m.call.* needs to be extended to support calls which add & remove streams mid-call - e.g. screen-share streams, mesh-based conferences, or simply adding video to a voice call. We should include metadata somehow about the streams that are being signalled so the other side knows how to present them (e.g. whether it's a screenshare, etc).

(Imported from https://matrix.org/jira/browse/SPEC-252)

(Reported by @ara4n)

[matrixbot]

Jira watchers: @ara4n

[zecakeh]

This seems to have been handled with MSC3077, merged in #1735?

[richvdh]

Agreed!