InstrumentPlayHandle: do not process if InstrumentTrack is muted

While regular instruments were excluded from processing when muted
this did not happen for InstrumentPlayHandle-based instruments. Muting
for exampling tracks with VSTi's inside did not decrease CPU usage.
Checking whether related InstrumentTrack is muted before calling
Instrument::play() fixes this issue.

Closes #2857426.
(cherry picked from commit 6940d19969)

src/core/Instrument.cpp

@@ -95,6 +95,15 @@ 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();