refacto: RenderingEngine::get_scene_manager() is now not callable from singleton

This permits to make evidence that we have some bad object passing on various code parts. I fixed majority of them to reduce the scope of passed objects

Unfortunately, for some edge cases i should have to expose ISceneManager from client, this should be fixed in the future when our POO will be cleaner client side (we have a mix of rendering and processing in majority of the client objects, it works but it's not clean)

src/client/client.h

@@ -355,6 +355,7 @@ public:
 	void setCamera(Camera* camera) { m_camera = camera; }
 
 	Camera* getCamera () { return m_camera; }
+	irr::scene::ISceneManager *getSceneManager();
 
 	bool shouldShowMinimap() const;