refacto: protect some RenderingEngine::get_scene_manager

* protect it from Camera, Sky, ClientMap object calls
* rename Game::sky to Game::m_sky

src/client/clientmap.h

@@ -68,6 +68,7 @@ class ClientMap : public Map, public scene::ISceneNode
 public:
 	ClientMap(
 			Client *client,
+			RenderingEngine *rendering_engine,
 			MapDrawControl &control,
 			s32 id
 	);