Cycles: update buffers only once in a while when progressive refine is used

It'll mimic the same behavior as regular tile rendering update and it should
give pretty noticeable boost on simpler scenes.

intern/cycles/render/session.h

@@ -179,6 +179,7 @@ protected:
 	double paused_time;
 
 	/* progressive refine */
+	double last_update_time;
 	bool update_progressive_refine(bool cancel);
 
 	vector<RenderBuffers *> tile_buffers;