Use the offsets for computing the private data offsets
Submitted by Emmanuele Bassi
GLib 2.38 changed the in-memory layout of instance and private data:
and introduces new API and macros to deal with this that make private data pointers obsolete.
Vala should switch to this new offset-based access, so that newly added API that relies on offsets (like the GTK template children API) can use it.