Handle for a [Resource]'s unique ID. The RID type is used to access the unique integer ID of a resource. They are opaque, which means they do not grant access to the associated resource by themselves. They are used by and with the low-level Server classes such as [VisualServer]. Creates a new RID instance with the ID of a given resource. When not handed a valid resource, silently stores the unused ID 0. Returns the ID of the referenced resource.