Add ResyncDownTracks API that can be used to resync all down tracks on (#3185)

* Add ResyncDownTracks API that can be used to resync all down tracks on
these receivers.

* actually call the function

pkg/sfu/redprimaryreceiver.go

@@ -121,6 +121,12 @@ func (r *RedPrimaryReceiver) DeleteDownTrack(subscriberID livekit.ParticipantID)
 	r.logger.Debugw("red primary receiver downtrack deleted", "subscriberID", subscriberID)
 }
 
+func (r *RedPrimaryReceiver) ResyncDownTracks() {
+	r.downTrackSpreader.Broadcast(func(dt TrackSender) {
+		dt.Resync()
+	})
+}
+
 func (r *RedPrimaryReceiver) IsClosed() bool {
 	return r.closed.Load()
 }

pkg/sfu/redreceiver.go

@@ -112,6 +112,12 @@ func (r *RedReceiver) DeleteDownTrack(subscriberID livekit.ParticipantID) {
 	r.logger.Debugw("red receiver downtrack deleted", "subscriberID", subscriberID)
 }
 
+func (r *RedReceiver) ResyncDownTracks() {
+	r.downTrackSpreader.Broadcast(func(dt TrackSender) {
+		dt.Resync()
+	})
+}
+
 func (r *RedReceiver) CanClose() bool {
 	return r.closed.Load() || r.downTrackSpreader.DownTrackCount() == 0
 }