Respect that cairo won't create image surfaces larger
than 32767 x 32767.
This makes the one reftest pass that specifically checks
this condition, treeview-crash-too-wide.
Language bindings—especially ones based on introspection—cannot deal
with custom type hiearchies. Luckily for us, GType has a derivable type
with low overhead: GTypeInstance.
By turning GskRenderNode into a GTypeInstance, and creating derived
types for each class of node, we can provide an introspectable API to
our non-C API consumers, with no functional change to the C API itself.
