Fix: Original music playback rate was slightly too fast

Found a new tempo value experimentally. The new value gives a timing error of less than 0.01 second per minute.

src/music/midifile.cpp

@@ -787,10 +787,11 @@ struct MpsMachine {
 	bool PlayInto()
 	{
 		/* Tempo seems to be handled as TEMPO_RATE = 148 ticks per second.
-		 * Use this as the tickdiv, and define the tempo to be one second (1M microseconds) per tickdiv.
+		 * Use this as the tickdiv, and define the tempo to be somewhat less than one second (1M microseconds) per quarter note.
+		 * This value was found experimentally to give a very close approximation of the correct playback speed.
 		 * MIDI software loading exported files will show a bogus tempo, but playback will be correct. */
 		this->target.tickdiv = TEMPO_RATE;
-		this->target.tempos.push_back(MidiFile::TempoChange(0, 1000000));
+		this->target.tempos.push_back(MidiFile::TempoChange(0, 980500));
 
 		/* Initialize playback simulation */
 		this->RestartSong();