add Evoral::SMF::end_track()

This changes the assumption that an SMF object only refers to
a single track when being written.

::end_track() is used within begin::write()/end_write() pairs to mark the start
of a new SMF track.

libs/evoral/SMF.cc

@@ -548,7 +548,15 @@ SMF::begin_write()
 	assert(_smf_track);
 
 	smf_add_track(_smf, _smf_track);
-	assert(_smf->number_of_tracks == 1);
+}
+
+void
+SMF::end_track ()
+{
+	/* nothing to do in libsmf API to "close" the track, it seems */
+	_smf_track = smf_track_new();
+	assert (_smf_track);
+	smf_add_track(_smf, _smf_track);
 }
 
 void

libs/evoral/evoral/SMF.h

@@ -168,6 +168,8 @@ public:
 	bool         _has_pgm_change;
 	int          _num_channels;
 	UsedChannels _used_channels;
+
+	void end_track ();
 };
 
 }; /* namespace Evoral */