Restrict negotiation of downstream codecs, allow multiple profiles.

We now restrict the allowable codecs in the downstream direction,
which leads to a clean failure instead of a silent track.  We also
allow multiple profiles for a single codec.

rtpconn/webclient.go

@@ -389,6 +389,26 @@ func addDownTrackUnlocked(conn *rtpDownConnection, remoteTrack *rtpUpTrack, remo
 		return err
 	}
 
+	codec := local.Codec()
+	ptype, err := group.CodecPayloadType(local.Codec())
+	if err != nil {
+		log.Printf("Couldn't determine ptype for codec %v: %v",
+			codec.MimeType, err)
+	} else {
+		err := transceiver.SetCodecPreferences(
+			[]webrtc.RTPCodecParameters{
+				{
+					RTPCodecCapability: codec,
+					PayloadType:        ptype,
+				},
+			},
+		)
+		if err != nil {
+			log.Printf("Couldn't set ptype for codec %v: %v",
+				codec.MimeType, err)
+		}
+	}
+
 	parms := transceiver.Sender().GetParameters()
 	if len(parms.Encodings) != 1 {
 		return errors.New("got multiple encodings")