Cycles / Tile Rendering:

* Added new option to chose the tile order.
In addition to the "Center" method, 4 new methods are available now, like Top -> Bottom and Right -> Left. 

Thanks to Sergey for code review and some tweaks!

intern/cycles/render/session.h

@@ -51,6 +51,7 @@ public:
 	bool experimental;
 	int samples;
 	int2 tile_size;
+	int tile_order;
 	int start_resolution;
 	int threads;