Implement this methods to handle gltf extra reading
Note that this method will be invoked every time an "extras" element will be found in the gltf file.
You can check the parentName to know where the "extras" element has been found.
loader - the GltfLoader with all the data structures
parentName - the name of the element being read
parent - the element being read
extras - the content of the extras found in the element being read
input - an object containing already loaded data from the element, this is most probably a JME object
An object of the same type as input, containing the data from the input object and the eventual additional data read from the extras