gsk: Mark some subsurface node apis as skip
These are not usable outside of GTK, so lets not burden bindings with them.
I'll keep the get_child() function exposed, since it is needed to iterate over node trees containing subsurface nodes.
These are not usable outside of GTK, so lets not burden bindings with them.
I'll keep the get_child() function exposed, since it is needed to iterate over node trees containing subsurface nodes.