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 [RenderingServer]. Constructs an empty [RID] with the invalid ID [code]0[/code]. Constructs a [RID] as a copy of the given [RID]. Returns the ID of the referenced resource.