Ensure that NotePlayHandles get processed before the InstrumentPlayHandle on instruments that use both NotePlayHandles and InstrumentPlayHandle, such as LB302 and SF2-Player

Issue: Currently, we use threads to process all PlayHandles, so there's no guarantee of the order they are processed in. This causes timing inaccuracy and jitter: notes of instruments that use both NPH's and IPH's can get randomly delayed by one entire period.
The issue is solved thusly:
- When processing an IPH, we check if the instrument is midi-based. If yes, we just process it normally (no NPH's to worry about).
- If it's not, then it also uses NPH's, so we'll have the IPH wait until all NPH's belonging to same instrument have been processed. There's some similar code in the new FX mixer, I pretty much just copied how we do it there.

plugins/sf2_player/sf2_player.h

@@ -79,7 +79,7 @@ public:
 
 	virtual Flags flags() const
 	{
-		return IsSingleStreamed | IsMidiBased;
+		return IsSingleStreamed;
 	}
 
 	virtual PluginView * instantiateView( QWidget * _parent );