Use layer 2 for SVC always. (#1542)

This fixes the case of screen share forwarding. We should probably also
look at proper AddTrack. The problem was that
- AddTrack used two layers for screen share from JS sample app
- Track was published with rid = f. Given that and the track info,
  consistent layer mapping set the layer as 1.
- `getBufferLocked` always uses the highest layer for SVC
- Between the two, when down track was requesting PLI, there was
  no buffer at the requested layer and hence no PLI went out.

A few other notes
- Tried locking SVC to layer 0 (instead of layer 2), but that resulted
  in PLI layer lock spamming. It did not happen in v1.3.0 of the server
  though. Not sure what causes that. Need to investigate later.
  But, that does not happen when using layer 2 buffer as SVC buffer.
- When using layer 2 for SVC, the PLI throttle config will be using that
  of layer 2. Is that okay?
- `buffer` structure should maintain more stats about spatial layers for
  SVC case so that layer stats can be reported to analytics/scoring etc.
- In general, `buffer` may need some more hooks to make it SVC aware so
  that it can handle various spetial layer aware/specific bits.

pkg/sfu/connectionquality/scorer.go

@@ -252,13 +252,14 @@ func (q *qualityScorer) Update(stat *windowStat, at time.Time) {
 		return
 	}
 
+	plw := q.getPacketLossWeight(stat)
 	reason := "none"
 	var score float64
 	if stat.packetsExpected == 0 {
 		reason = "dry"
 		score = poorScore
 	} else {
-		packetScore := stat.calculatePacketScore(q.getPacketLossWeight(stat), q.params.IsDependentRTT, q.params.IsDependentJitter)
+		packetScore := stat.calculatePacketScore(plw, q.params.IsDependentRTT, q.params.IsDependentJitter)
 		bitrateScore := stat.calculateBitrateScore(expectedBitrate)
 		layerScore := math.Max(math.Min(maxScore, maxScore-(expectedDistance*distanceWeight)), 0.0)
 
@@ -302,6 +303,8 @@ func (q *qualityScorer) Update(stat *windowStat, at time.Time) {
 			"score", score,
 			"quality", scoreToConnectionQuality(score),
 			"stat", stat,
+			"packetLossWeight", plw,
+			"maxPPS", q.maxPPS,
 			"expectedBitrate", expectedBitrate,
 			"expectedDistance", expectedDistance,
 		)

pkg/sfu/receiver.go

@@ -298,7 +298,12 @@ func (w *WebRTCReceiver) AddUpTrack(track *webrtc.TrackRemote, buff *buffer.Buff
 	}
 
 	layer := int32(0)
-	if w.Kind() == webrtc.RTPCodecTypeVideo {
+	// for svc codecs, use layer full quality instead.
+	// we only have buffer for full quality
+	if w.isSVC {
+		layer = int32(len(w.buffers)) - 1
+	}
+	if w.Kind() == webrtc.RTPCodecTypeVideo && !w.isSVC {
 		layer = buffer.RidToSpatialLayer(track.RID(), w.trackInfo)
 	}
 	buff.SetLogger(w.logger.WithValues("layer", layer))