Implement pid and mark rewriting for VP9, fix VP9 sync bits.

The sync bits were incorrect.  In addition, we need to set the
marker at the new end of the frame when doing spatial decimation.

rtpconn/rtpconn.go

@@ -248,9 +248,23 @@ func (down *rtpDownTrack) Write(buf []byte) (int, error) {
 	}
 
 	if flags.Start && (layer.sid != layer.wantedSid) {
-		if flags.SidSync {
-			layer.sid = layer.wantedSid
-			down.setLayerInfo(layer)
+		if layer.wantedSid < layer.sid {
+			if flags.Keyframe {
+				layer.sid = layer.wantedSid
+				down.setLayerInfo(layer)
+			} else {
+				down.remote.RequestKeyframe()
+			}
+		} else if layer.wantedSid > layer.sid {
+			if flags.Keyframe {
+				layer.sid = layer.wantedSid
+				down.setLayerInfo(layer)
+			} else if flags.SidUpSync {
+				layer.sid = layer.sid + 1
+				down.setLayerInfo(layer)
+			} else {
+				down.remote.RequestKeyframe()
+			}
 		}
 	}
 
@@ -267,7 +281,9 @@ func (down *rtpDownTrack) Write(buf []byte) (int, error) {
 		return 0, nil
 	}
 
-	if newseqno == flags.Seqno && piddelta == 0 {
+	setMarker := flags.Sid == layer.sid && flags.End && !flags.Marker
+
+	if !setMarker && newseqno == flags.Seqno && piddelta == 0 {
 		return down.write(buf)
 	}
 
@@ -276,7 +292,7 @@ func (down *rtpDownTrack) Write(buf []byte) (int, error) {
 	buf2 := ibuf2.([]byte)
 
 	n := copy(buf2, buf)
-	err = codecs.RewritePacket(codec, buf2[:n], newseqno, piddelta)
+	err = codecs.RewritePacket(codec, buf2[:n], setMarker, newseqno, piddelta)
 	if err != nil {
 		return 0, err
 	}