Fix Clementine getting stuck when transitioning from a local track to a Spotify track with crossfade disabled.

Two problems here:
- the first was that "StartPlaybackLater" wasn't called from the thread which created SpotifyServer, so the timer never started.
- then the playback sometimes failed or started with an offset: just hack to ignore sourcedrained signal in this case.

src/engines/gstenginepipeline.cpp

@@ -174,8 +174,12 @@ bool GstEnginePipeline::ReplaceDecodeBin(const QUrl& url) {
     gst_object_unref(GST_OBJECT(pad));
 
     // Tell spotify to start sending data to us.
-    InternetModel::Service<SpotifyService>()->server()->StartPlaybackLater(
+    SpotifyServer* spotify_server = InternetModel::Service<SpotifyService>()->server();
-        url.toString(), port);
+    // Need to schedule this in the spotify server's thread
+    QMetaObject::invokeMethod(spotify_server, "StartPlayback",
+                              Qt::QueuedConnection,
+                              Q_ARG(QString, url.toString()),
+                              Q_ARG(quint16, port));
   } else {
     new_bin = engine_->CreateElement("uridecodebin");
     g_object_set(G_OBJECT(new_bin), "uri", url.toEncoded().constData(),
@@ -920,7 +924,14 @@ void GstEnginePipeline::SourceDrainedCallback(GstURIDecodeBin* bin,
                                               gpointer self) {
   GstEnginePipeline* instance = reinterpret_cast<GstEnginePipeline*>(self);
 
-  if (instance->has_next_valid_url()) {
+  if (instance->has_next_valid_url() &&
+      // I'm not sure why, but calling this when previous track is a local song
+      // and the next track is a Spotify song is buggy: the Spotify song will
+      // not start or with some offset. So just do nothing here: when the song
+      // finished, EndOfStreamReached/TrackEnded will be emitted anyway so
+      // NextItem will be called.
+      !(instance->url_.scheme() != "spotify" &&
+        instance->next_url_.scheme() == "spotify")) {
     instance->TransitionToNext();
   }
 }

src/internet/spotify/spotifyserver.cpp

@@ -260,15 +260,6 @@ void SpotifyServer::RemoveSongsFromPlaylist(
   SendOrQueueMessage(message);
 }
 
-void SpotifyServer::StartPlaybackLater(const QString& uri, quint16 port) {
-  QTimer* timer = new QTimer(this);
-  connect(timer, SIGNAL(timeout()), timer, SLOT(deleteLater()));
-
-  timer->start(100);  // lol
-  NewClosure(timer, SIGNAL(timeout()), this,
-             SLOT(StartPlayback(QString, quint16)), uri, port);
-}
-
 void SpotifyServer::StartPlayback(const QString& uri, quint16 port) {
   pb::spotify::Message message;
   pb::spotify::PlaybackRequest* req = message.mutable_playback_request();

src/internet/spotify/spotifyserver.h

@@ -53,7 +53,6 @@ class SpotifyServer : public AbstractMessageHandler<pb::spotify::Message> {
   void RemoveSongsFromUserPlaylist(int playlist_index,
                                    const QList<int>& songs_indices_to_remove);
   void RemoveSongsFromStarred(const QList<int>& songs_indices_to_remove);
-  void StartPlaybackLater(const QString& uri, quint16 port);
   void Search(const QString& text, int limit, int limit_album = 0);
   void LoadImage(const QString& id);
   void AlbumBrowse(const QString& uri);