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.cpp

@@ -1475,6 +1475,11 @@ bool Client::updateWieldedItem()
 	return true;
 }
 
+irr::scene::ISceneManager* Client::getSceneManager()
+{
+	return m_rendering_engine->get_scene_manager();
+}
+
 Inventory* Client::getInventory(const InventoryLocation &loc)
 {
 	switch(loc.type){