This structure is always allocated by GStreamer, can't be
subclassed or extended, and is never allocated or used on
the stack, so we don't need any padding and can extend it
as we please.
It was a bit too clever, and didn't really work as an API,
confusing people to no end. Better implement specific methods
whether an interface is usable/available/ready on the interface
itself, or even add GError arguments, rather than try to have
per-instance interfaces.
