SVC with RID -> spatial layer mapping (#3754)

* SVC with RID -> spatial layer mapping

There are cases where an SVC track comes in with a RID.
As there is no RID announced in SDP, it maps to invalid layer.
Seems to happen with older browsers.

* test

pkg/sfu/buffer/videolayerutils.go

@@ -351,6 +351,19 @@ func GetSpatialLayerForRid(rid string, ti *livekit.TrackInfo) int32 {
 	if len(ti.Layers) == 1 {
 		// single layer without RID
 		return 0
+	} else if len(ti.Layers) > 1 {
+		// RID present in codec, but not specified via signalling
+		// (happens with older browsers setting a rid for SVC codecs)
+		hasRid := false
+		for _, layer := range ti.Layers {
+			if layer.Rid != "" {
+				hasRid = true
+				break
+			}
+		}
+		if !hasRid {
+			return 0
+		}
 	}
 
 	return InvalidLayerSpatial