Instrument: also render sound for MIDI-based instruments when muted

In order to provide smooth muting functionality (i.e. immediate proper
sound when unmuting) always render audio buffers for MIDI-based
instruments. This is more important than potentially reduced CPU usage
while muted.

Closes #69.

src/core/Instrument.cpp

@@ -96,14 +96,6 @@ bool Instrument::isFromTrack( const track * _track ) const
 
 
 
-bool Instrument::isMuted() const
-{
-	return m_instrumentTrack->isMuted();
-}
-
-
-
-
 void Instrument::applyRelease( sampleFrame * buf, const notePlayHandle * _n )
 {
 	const fpp_t frames = _n->framesLeftForCurrentPeriod();