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.

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)