Revision of handling of frameoffset for NPH, SPH

Change in handling of frameoffset for multistreamed instruments and sampletracks.
- Instead of holding the offset for the lifetime of the playhandle, negate the offset in the first period
- Multistream-instruments require some small changes: they have to now check for the offset and accordingly leave empty space in the start of the period (already done in this commit)
- There are possibly optimizations that can be done later
- This change is necessary so that we can have sample-exact models, and sample-exact vol/pan knobs for all instruments. Earlier multistream instruments were always rendering some frames ahead-of-time, so applying sample-exact data for them would have been impossible, since we don't have the future-values yet...

src/tracks/InstrumentTrack.cpp

@@ -193,26 +193,22 @@ void InstrumentTrack::processAudioBuffer( sampleFrame* buf, const fpp_t frames,
 	// is no problem for us since we just bypass the envelopes+LFOs
 	if( m_instrument->flags().testFlag( Instrument::IsSingleStreamed ) == false && n != NULL )
 	{
-		m_soundShaping.processAudioBuffer( buf, frames, n );
+		const f_cnt_t offset = n->noteOffset();
+		m_soundShaping.processAudioBuffer( buf + offset, frames - offset, n );
 		v_scale *= ( (float) n->getVolume() / DefaultVolume );
 	}
 
 	m_audioPort.setNextFxChannel( m_effectChannelModel.value() );
 	
-	int framesToMix = frames;
-	int offset = 0;
 	int panning = m_panningModel.value();
 
 	if( n )
 	{
-		framesToMix = qMin<f_cnt_t>( n->framesLeftForCurrentPeriod(), framesToMix );
-		offset = n->offset();
-
 		panning += n->getPanning();
 		panning = tLimit<int>( panning, PanningLeft, PanningRight );
 	}
 
-	engine::mixer()->bufferToPort( buf, framesToMix, offset, panningToVolumeVector( panning, v_scale ), &m_audioPort );
+	engine::mixer()->bufferToPort( buf, frames, panningToVolumeVector( panning, v_scale ), &m_audioPort );
 }