change-codecs: move codec selection to the receiver

While using setCodecPreferences on the sending side works because
the answerer will match the sender's order, using it on the receiving
end is more semantically correct.

See this comment
w3c/webrtc-encoded-transform#186 (comment)

src/content/peerconnection/change-codecs/js/main.js

@@ -112,21 +112,6 @@ async function call() {
 
  localStream.getTracks().forEach(track => pc1.addTrack(track, localStream));
  console.log('Added local stream to pc1');
- if (supportsSetCodecPreferences) {
- const preferredCodec = codecPreferences.options[codecPreferences.selectedIndex];
- if (preferredCodec.value !== '') {
- const [mimeType, sdpFmtpLine] = preferredCodec.value.split(' ');
- const {codecs} = RTCRtpSender.getCapabilities('video');
- const selectedCodecIndex = codecs.findIndex(c => c.mimeType === mimeType && c.sdpFmtpLine === sdpFmtpLine);
- const selectedCodec = codecs[selectedCodecIndex];
- codecs.splice(selectedCodecIndex, 1);
- codecs.unshift(selectedCodec);
- console.log(codecs);
- const transceiver = pc1.getTransceivers().find(t => t.sender && t.sender.track === localStream.getVideoTracks()[0]);
- transceiver.setCodecPreferences(codecs);
- console.log('Preferred video codec', selectedCodec);
- }
- }
  codecPreferences.disabled = true;
 
  try {
@@ -185,6 +170,21 @@ function onSetSessionDescriptionError(error) {
 }
 
 function gotRemoteStream(e) {
+ // Set codec preferences on the receiving side.
+ if (supportsSetCodecPreferences) {
+ const preferredCodec = codecPreferences.options[codecPreferences.selectedIndex];
+ if (preferredCodec.value !== '') {
+ const [mimeType, sdpFmtpLine] = preferredCodec.value.split(' ');
+ const {codecs} = RTCRtpReceiver.getCapabilities('video');
+ const selectedCodecIndex = codecs.findIndex(c => c.mimeType === mimeType && c.sdpFmtpLine === sdpFmtpLine);
+ const selectedCodec = codecs[selectedCodecIndex];
+ codecs.splice(selectedCodecIndex, 1);
+ codecs.unshift(selectedCodec);
+ e.transceiver.setCodecPreferences(codecs);
+ console.log('Preferred video codec', selectedCodec);
+ }
+ }
+
  if (remoteVideo.srcObject !== e.streams[0]) {
  remoteVideo.srcObject = e.streams[0];
  console.log('pc2 received remote stream');