Instead of references. This is needed because those function pointers
are used in GDNative which needs to work with plain C, which doesn't
support passing parameters by reference.
The validated getters were only setting the value without changing the
type, leading to wrong results. This uses the same path used for
methods to the same purpose.
The underscore prefix was used to avoid the conflict between the `RID` class
name and the matching enum value in `Variant::Type`.
This can be fixed differently by prefixing uses of the `RID` class in `Variant`
with the scope resolution operator, as done already for `AABB`.
