Don't let silent instruments wake up sleeping effects

[DomClark]

Fixes #4564.

Ensures a PlayHandle's buffer is not silent before mixing it in to the audio port. This avoids waking up all downstream effects unless there is some audio present. The test is not performed for NotePlayHandles for performance reasons, in line with similar code in InstrumentTrack::processAudioBuffer.

lmms/src/tracks/InstrumentTrack.cpp

Lines 170 to 175 in c1ae1ed

	// Test for silent input data if instrument provides a single stream only (i.e. driven by InstrumentPlayHandle)
	// We could do that in all other cases as well but the overhead for silence test is bigger than
	// what we potentially save. While playing a note, a NotePlayHandle-driven instrument will produce sound in
	// 99 of 100 cases so that test would be a waste of time.
	if( m_instrument->flags().testFlag( Instrument::IsSingleStreamed ) &&
	MixHelpers::isSilent( buf, frames ) )

Using a slightly different approach from #4564 (comment) as suggested by gi0e5b06.

[T0NIT0RMX]

I tried it and it works great ! Definetly fixes #4564.

[PhysSong]

Merge?