InstrumentTrack: always process silence at least one time

As of commit 1266278229 silent buffers
from InstrumentPlayHandle-driven instruments are ignored. This is a good
thing but can beak PeakController instances attached to e.g. ZynAddSubFX.

Fixes playback of unfa-Spoken.mmpz.

include/InstrumentTrack.h

@@ -229,6 +229,8 @@ private:
 	int m_runningMidiNotes[NumKeys];
 	bool m_sustainPedalPressed;
 
+	bool m_silentBuffersProcessed;
+
 	IntModel m_baseNoteModel;
 
 	NotePlayHandleList m_processHandles;