gtk4: use impl_offset() for calculating template child offset
Since the #[repr(rust)] layout of PrivateStruct could change, this was
failing to take the offset of `imp` into account.
@@ -1216,7 +1216,7 @@ pub unsafe trait WidgetClassSubclassExt: ClassStruct {
let widget_class = self as *mut _ as *mut ffi::GtkWidgetClass;
let private_offset = <Self::Type as ObjectSubclassType>::type_data()
.as_ref()
- .private_offset;
+ .impl_offset();
ffi::gtk_widget_class_bind_template_child_full(
widget_class,
name.to_glib_none().0,
