Implement renegotiation of down streams.

We used to destroy and recreate down streams whenever something changed,
which turned out to be racy.  We now properly implement renegotiation,
as well as atomic replacement of a stream by another one.

rtpconn/rtpconn.go

@@ -78,6 +78,7 @@ type downTrackAtomics struct {
 
 type rtpDownTrack struct {
 	track      *webrtc.TrackLocalStaticRTP
+	sender     *webrtc.RTPSender
 	remote     conn.UpTrack
 	ssrc       webrtc.SSRC
 	maxBitrate *bitrate
@@ -156,7 +157,7 @@ func (down *rtpDownConnection) getTracks() []*rtpDownTrack {
 }
 
 func newDownConn(c group.Client, id string, remote conn.Up) (*rtpDownConnection, error) {
-	api := group.APIFromCodecs(remote.Codecs())
+	api := c.Group().API()
 	pc, err := api.NewPeerConnection(*ice.ICEConfiguration())
 	if err != nil {
 		return nil, err
@@ -366,17 +367,6 @@ func (up *rtpUpConnection) User() (string, string) {
 	return up.userId, up.username
 }
 
-func (up *rtpUpConnection) Codecs() []webrtc.RTPCodecCapability {
-	up.mu.Lock()
-	defer up.mu.Unlock()
-
-	codecs := make([]webrtc.RTPCodecCapability, len(up.tracks))
-	for i := range up.tracks {
-		codecs[i] = up.tracks[i].Codec()
-	}
-	return codecs
-}
-
 func (up *rtpUpConnection) AddLocal(local conn.Down) error {
 	up.mu.Lock()
 	defer up.mu.Unlock()