- Allow getting an opaque pointer, no matter the type (for ptrcall).
- Allow setting object pointer and id directly.
- Allow initializing the data given a type, to allow properly setting
return types on ptrcalls.
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`.
