Fix incorrectly saved un-used playlists

This addresses issues with session-cleanup and region-cleanup in
some sessions.

The root-cause why some unused playlists were saved in the session XML
under <Playlists> and not <UnusedPlaylists> is not known.

Early 6.0-pre did incorrect reference counting, but also older sessions
had this issue. Perhaps due to ambiguities of matching playlists
by name in 5.x or session-format changes 3.x .. 5.x.

libs/ardour/ardour/session_playlists.h

@@ -72,6 +72,7 @@ private:
 	void remove (boost::shared_ptr<Playlist>);
 	void remove_weak (boost::weak_ptr<Playlist>);
 	void track (bool, boost::weak_ptr<Playlist>);
+	void update_tracking ();
 
 	void find_equivalent_playlist_regions (boost::shared_ptr<Region>, std::vector<boost::shared_ptr<Region> >& result);
 	void update_after_tempo_map_change ();

libs/ardour/session_playlists.cc

@@ -111,6 +111,30 @@ SessionPlaylists::remove (boost::shared_ptr<Playlist> playlist)
 	}
 }
 
+void
+SessionPlaylists::update_tracking ()
+{
+	/* This is intended to be called during session-load, after loading
+	 * playlists and re-assigning them to tracks (refcnt is up to date).
+	 * Check playlist refcnt, move unused playlist to unused_playlists
+	 * array (which may be the case when loading old sessions)
+	 */
+	for (List::iterator i = playlists.begin(); i != playlists.end(); ) {
+		if ((*i)->hidden () || (*i)->used ()) {
+			++i;
+			continue;
+		}
+
+		warning << _("Session State: Unused playlist was listed as used.") << endmsg;
+
+		assert (unused_playlists.find (*i) == unused_playlists.end());
+		unused_playlists.insert (*i);
+
+		List::iterator rm = i;
+		++i;
+		 playlists.erase (rm);
+	}
+}
 
 void
 SessionPlaylists::track (bool inuse, boost::weak_ptr<Playlist> wpl)

libs/ardour/session_state.cc

@@ -1702,6 +1702,9 @@ Session::set_state (const XMLNode& node, int version)
 		goto out;
 	}
 
+	/* Now that we Tracks have been loaded and playlists are assigned */
+	_playlists->update_tracking ();
+
 	/* Now that we have Routes and masters loaded, connect them if appropriate */
 
 	Slavable::Assign (_vca_manager); /* EMIT SIGNAL */