InstrumentTrack: scale output volume by 2 for MIDI-based instruments

As of 0ca3901ab8 MIDI-based instruments are
played at velocity 63 for volume=100%.  In order to get the same output
volume, we need to scale it by 2.

src/tracks/InstrumentTrack.cpp

@@ -185,6 +185,13 @@ void InstrumentTrack::processAudioBuffer( sampleFrame* buf, const fpp_t frames,
 
 	float v_scale = (float) getVolume() / DefaultVolume;
 
+	// We play MIDI-based instruments at velocity 63 for volume=100%. In order
+	// to get the same output volume, we need to scale it by 2
+	if( m_instrument->flags().testFlag( Instrument::IsMidiBased ) )
+	{
+		v_scale *= 2;
+	}
+
 	// instruments using instrument-play-handles will call this method
 	// without any knowledge about notes, so they pass NULL for n, which
 	// is no problem for us since we just bypass the envelopes+LFOs