Made DifferenceType enum private

Author: fuzun

strobe/strobe-api.h

@@ -83,15 +83,15 @@ class StrobeAPI
 		_unassigned4 = (PHASE_POSITIVE | PHASE_INVERTED | FRAME_RENDER)
 	};
 
+private:
+
 	enum DifferenceType
 	{
 		PositiveDifference,
 		NegativeDifference,
 		TotalDifference
 	};
 
-private:
-	
 	struct Counter
 	{
 		int positiveRenderedFrameCount;