refacto: don't use RenderingEngine singleton on CAO

* we don't need on CAO side more than SceneManager, and temporary. Pass only required SceneManager as a parameter to build CAO and add them to the current scene
* Use temporary the RenderingEngine singleton from ClientEnvironment, waitfor for better solution
* Make ClientActiveObject::addToScene virtual function mandatory to be defined by children to ensure we don't forget to properly define it

src/unittest/test_clientactiveobjectmgr.cpp

@@ -29,6 +29,7 @@ public:
 	TestClientActiveObject() : ClientActiveObject(0, nullptr, nullptr) {}
 	~TestClientActiveObject() = default;
 	ActiveObjectType getType() const { return ACTIVEOBJECT_TYPE_TEST; }
+	virtual void addToScene(ITextureSource *tsrc, irr::scene::ISceneManager *smgr) {}
 };
 
 class TestClientActiveObjectMgr : public TestBase