Request key frame on subscription change. (#3349)

To assist the path where the requested layer is higher than current seen
maximum and there is no congestion.

pkg/sfu/downtrack.go

@@ -1244,6 +1244,7 @@ func (d *DownTrack) SetMaxSpatialLayer(spatialLayer int32) {
 	}
 
 	d.postMaxLayerNotifierEvent("max-subscribed")
+	d.postKeyFrameRequestEvent()
 
 	if sal := d.getStreamAllocatorListener(); sal != nil {
 		sal.OnSubscribedLayerChanged(d, maxLayer)