This function registers a font file loader with DirectWrite. The font file loader interface, which should be implemented by a singleton object, handles loading font file resources of a particular type from a key. A given instance can only be registered once. Succeeding attempts will return an error, indicating that it has already been registered. Note that font file loader implementations must not register themselves with DirectWrite inside their constructors, and must not unregister themselves inside their destructors, because registration and unregistraton operations increment and decrement the object reference count respectively. Instead, registration and unregistration with DirectWrite of font file loaders should be performed outside of the font file loader implementation.