refacto: hide mesh_cache inside the rendering engine

This permit cleaner access to meshCache and ensure we don't access to it from all the code

src/client/client.cpp

@@ -1983,7 +1983,7 @@ scene::IAnimatedMesh* Client::getMesh(const std::string &filename, bool cache)
 		return nullptr;
 	mesh->grab();
 	if (!cache)
-		m_rendering_engine->get_mesh_cache()->removeMesh(mesh);
+		m_rendering_engine->removeMesh(mesh);
 	return mesh;
 }