Use mids instead of track ids for indexing labels.

It turns out that track ids are not necessarily the same on the local and
remote sides.  Thanks to Ines Klimann for noticing the issue.

static/sfu.js

@@ -41,6 +41,7 @@ function Connection(id, pc) {
     this.pc = pc;
     this.stream = null;
     this.labels = {};
+    this.labelsByMid = {};
     this.iceCandidates = [];
     this.timers = [];
     this.audioStats = {};
@@ -647,12 +648,18 @@ async function gotOffer(id, labels, offer) {
         };
 
         c.pc.ontrack = function(e) {
+            let label = e.transceiver && c.labelsByMid[e.transceiver.mid];
+            if(label) {
+                c.labels[e.track.id] = label;
+            } else {
+                console.error("Couldn't find label for track");
+            }
             c.stream = e.streams[0];
             setMedia(id);
         };
     }
 
-    c.labels = labels;
+    c.labelsByMid = labels;
 
     await c.pc.setRemoteDescription(offer);
     await addIceCandidates(c);
@@ -1066,10 +1073,22 @@ async function negotiate(id) {
     if(!offer)
         throw(new Error("Didn't create offer"));
     await c.pc.setLocalDescription(offer);
+
+    // mids are not known until this point
+    c.pc.getTransceivers().forEach(t => {
+        if(t.sender && t.sender.track) {
+            let label = c.labels[t.sender.track.id];
+            if(label)
+                c.labelsByMid[t.mid] = label;
+            else
+                console.error("Couldn't find label for track");
+        }
+    });
+
     send({
         type: 'offer',
         id: id,
-        labels: c.labels,
+        labels: c.labelsByMid,
         offer: offer,
     });
 }