Fix track info available for down tracks. (#967)

Should have here in the first place. Brain damage :-(

pkg/rtc/wrappedreceiver.go

@@ -113,7 +113,6 @@ func (d *DummyReceiver) Upgrade(receiver sfu.TrackReceiver) {
 	d.downtrackLock.Lock()
 	for _, t := range d.downtracks {
 		receiver.AddDownTrack(t)
-		t.TrackInfoAvailable()
 	}
 	d.downtracks = make(map[livekit.ParticipantID]sfu.TrackSender)
 	d.downtrackLock.Unlock()

pkg/sfu/receiver.go

@@ -386,6 +386,7 @@ func (w *WebRTCReceiver) AddDownTrack(track TrackSender) error {
 			track.UpTrackLayersChange(layers)
 		}
 	}
+	track.TrackInfoAvailable()
 
 	w.downTrackSpreader.Store(track)
 	return nil