[cairo] surfaces and backends

[Steve Chaplin]

I'd like to implement some new surface methods for pycairo
surface.set_size()
surface.set_device_offset()

They're available for the xlib backend, but not other backends, so I would
need to check that the surface is an xlib surface.
Does Cairo provide a direct way to ask a surface which backend it was created for?
Or is that something a language binding should handle itself (if needed) using
cairo_surface_set_user_data()?

Steve