RenderEngine/Nodes: system to check for shading nodes compatibility

* Scene.use_shading_nodes property to check if RenderEngine is using new shading
  nodes system, and RenderEngine.bl_use_shading_nodes to set this.
* Add mechanism for tagging nodes as being compatible with the old/new system.

source/blender/render/extern/include/RE_engine.h

@@ -51,6 +51,7 @@ struct Scene;
 #define RE_GAME					2
 #define RE_USE_PREVIEW			4
 #define RE_USE_POSTPROCESS		8
+#define RE_USE_SHADING_NODES	16
 
 /* RenderEngine.flag */
 #define RE_ENGINE_ANIMATION		1