avmerge: Close streams to avoid errors while changing quality

If a fetchRange network request finished after the quality was
changed, there would be a "InvalidStateError: An attempt was made
to use an object that is not, or is no longer, usable" because
appendSegment was trying to append to the sourceBuffer that was
unusable after the video src was changed to a new mediaSource.

Adds a close method to the AVMerge class to properly clean and
close everything so these sorts of errors won't happen.

Signed-off-by: Jesús <heckyel@hyperbola.info>

youtube/templates/watch.html

@@ -119,6 +119,8 @@
                          var videoPaused = video.paused;
                          var videoSpeed = video.playbackRate;
                          var videoSource;
+                         if (avMerge)
+                             avMerge.close();
                          if (selection.type == 'uni'){
                              videoSource = data['uni_sources'][selection.index];
                              video.src = videoSource.url;