Fix screenshare failing after several attempts (#2771)
* Fix screenshare failing after several attempts
Re-use any existing transceivers when screen sharing. This prevents
transceivers accumulating and making the SDP too big: see linked bug.
This also switches from `addTrack()` to `addTransceiver ()` which is
not that large of a change, other than having to explicitly find the
transceivers after an offer has arrived rather than just adding tracks
and letting WebRTC take care of it.
Fixes https://github.com/vector-im/element-call/issues/625
* Fix tests
* Unused import
* Use a map instead of an array
* Add comment
* more comment
* Remove commented code
* Remove unintentional debugging
* Add test for screenshare transceiver re-use
* Type alias for transceiver map
