This merged gtk, gdk and gsk into one library, making it possible to
have internal private APIs between gtk them, as well as producing more
efficient code.
https://bugzilla.gnome.org/show_bug.cgi?id=773100
The child-transform is useful only if we also provide clipping to the
parent nodes, otherwise children will just be drawn outside of the
parent's bounds.
We'll introduce child transforms either at a higher layer, or once we
add clipping support to GskRenderNode.