Fix updating of monitors.

 * It was redrawing the HTML each time the graphs updated, instead of
just drawing the canvas, which meant the handlers were getting missed,
as they were sometimes on, sometimes off.

src/components/monitors/monitors.js

@@ -63,7 +63,7 @@ export default class Monitors {
 
   onTracksChanged(tracks) {
     this.tracks = tracks;
-    this.refresh();
+    this.renderMonitors();
   }
 
   onTrackChanged() {