Fixed a bug in how the demo detects audio assets.

In HLS, the resolution property is optional. This means we
cannot assume that a variant with a video stream has a defined
width. Such a check was causing us to incorrectly identify
variants as audio-only, and applying the audio-only poster
incorrectly.
Instead, this change checks for the presence of a video codec.

Closes #794

Change-Id: I80ebd11ebc735468220816c5133a3398436f4530

demo/asset_section.js

@@ -202,8 +202,9 @@ shakaDemo.load = function() {
     shakaDemo.hashShouldChange_();
 
     // Audio-only tracks have no width/height.
-    var videoTracks =
-        player.getVariantTracks().filter(function(t) { return t.width; });
+    var videoTracks = player.getVariantTracks().filter(function(t) {
+      return t.videoCodec;
+    });
 
     // Set a different poster for audio-only assets.
     if (videoTracks.length == 0) {

demo/receiver_app.js

@@ -128,8 +128,9 @@ ShakaReceiver.prototype.checkIdle_ = function() {
     this.cancelIdleTimer_();
 
     // Audio-only tracks have no width/height.
-    var videoTracks =
-        this.player_.getVariantTracks().filter(function(t) { return t.width; });
+    var videoTracks = this.player_.getVariantTracks().filter(function(t) {
+      return t.videoCodec;
+    });
 
     // Set a special poster for audio-only assets.
     if (videoTracks.length == 0) {