Populate MediaEngine from Offer in broadcast

Use offerer PayloadType when generating answer. Before we would use the
hardcoded values, this is invalid behavior and would cause browsers to
just ignore our video when publishing.

README.md

@@ -144,6 +144,7 @@ Check out the **[contributing wiki](https://github.com/pion/webrtc/wiki/Contribu
 * [Quentin Renard](https://github.com/asticode)
 * [opennota](https://github.com/opennota)
 * [Simon Eisenmann](https://github.com/longsleep)
+* [Ben Weitzman](https://github.com/benweitzman)
 
 ### License
 MIT License - see [LICENSE](LICENSE) for full text

examples/broadcast/README.md

@@ -10,7 +10,7 @@ go get github.com/pion/webrtc/examples/broadcast
 ```
 
 ### Open broadcast example page
-[jsfiddle.net](https://jsfiddle.net/zhpya3n9/) You should see two buttons 'Publish a Broadcast' and 'Join a Broadcast'
+[jsfiddle.net](https://jsfiddle.net/d2mt13y6/) You should see two buttons 'Publish a Broadcast' and 'Join a Broadcast'
 
 ### Run Broadcast
 #### Linux/macOS

examples/broadcast/jsfiddle/demo.js

@@ -27,7 +27,7 @@ window.createSession = isPublisher => {
           .catch(log)
       }).catch(log)
   } else {
-    pc.addTransceiver('video', {'direction': 'recvonly'})
+    pc.addTransceiver('video')
     pc.createOffer()
       .then(d => pc.setLocalDescription(d))
       .catch(log)

examples/broadcast/main.go

@@ -19,20 +19,20 @@ func main() {
 	sdpChan := signal.HTTPSDPServer()
 
 	// Everything below is the Pion WebRTC API, thanks for using it ❤️.
-	// Create a MediaEngine object to configure the supported codec
-	m := webrtc.MediaEngine{}
-
-	// Setup the codecs you want to use.
-	// Only support VP8, this makes our proxying code simpler
-	m.RegisterCodec(webrtc.NewRTPVP8Codec(webrtc.DefaultPayloadTypeVP8, 90000))
-
-	// Create the API object with the MediaEngine
-	api := webrtc.NewAPI(webrtc.WithMediaEngine(m))
-
 	offer := webrtc.SessionDescription{}
 	signal.Decode(<-sdpChan, &offer)
 	fmt.Println("")
 
+	// Since we are answering use PayloadTypes declared by offerer
+	mediaEngine := webrtc.MediaEngine{}
+	err := mediaEngine.PopulateFromSDP(offer)
+	if err != nil {
+		panic(err)
+	}
+
+	// Create the API object with the MediaEngine
+	api := webrtc.NewAPI(webrtc.WithMediaEngine(mediaEngine))
+
 	peerConnectionConfig := webrtc.Configuration{
 		ICEServers: []webrtc.ICEServer{
 			{