Don't download segments while switching playlists

If bandwidth drops off precipitously, it's not a good idea to fetch a higher quality segment while waiting for the new media playlist to load. Delay segment loading instead. Less fetching will be done in parallel after this change but:
 - the player will adjust to dramatic bandwidth drops more quickly
 - the player will upshift more quickly when bandwidth returns

@@ -584,6 +584,11 @@ videojs.Hls.prototype.fillBuffer = function(offset) {
     return;
     return;
   }
   }
 
 
+  // if a playlist switch is in progress, wait for it to finish
+  if (this.playlists.state === 'SWITCHING_MEDIA') {
+    return;
+  }
+
   // if the video has finished downloading, stop trying to buffer
   // if the video has finished downloading, stop trying to buffer
   segment = this.playlists.media().segments[this.mediaIndex];
   segment = this.playlists.media().segments[this.mediaIndex];
   if (!segment) {
   if (!segment) {

@@ -976,6 +976,35 @@ test('only appends one segment at a time', function() {
   equal(appends, 0, 'did not append while updating');
   equal(appends, 0, 'did not append while updating');
 });
 });
 
 
+test('waits to download new segments until the media playlist is stable', function() {
+  var media;
+  player.src({
+    src: 'manifest/master.m3u8',
+    type: 'application/vnd.apple.mpegurl'
+  });
+  openMediaSource(player);
+  standardXHRResponse(requests.shift()); // master
+  player.hls.bandwidth = 1; // make sure we stay on the lowest variant
+  standardXHRResponse(requests.shift()); // media
+
+  // mock a playlist switch
+  media = player.hls.playlists.media();
+  player.hls.playlists.media = function() {
+    return media;
+  };
+  player.hls.playlists.state = 'SWITCHING_MEDIA';
+
+  standardXHRResponse(requests.shift()); // segment 0
+
+  equal(requests.length, 0, 'no requests outstanding');
+  player.hls.checkBuffer_();
+  equal(requests.length, 0, 'delays segment fetching');
+
+  player.hls.playlists.state = 'LOADED_METADATA';
+  player.hls.checkBuffer_();
+  equal(requests.length, 1, 'resumes segment fetching');
+});
+
 test('cancels outstanding XHRs when seeking', function() {
 test('cancels outstanding XHRs when seeking', function() {
   player.src({
   player.src({
     src: 'manifest/media.m3u8',
     src: 'manifest/media.m3u8',