cairo-qt-surface: fix arguments of _cairo_surface_init()
Split out from !37 (merged) as per @psychon's comment
The function signature is
cairo_private void
_cairo_surface_init (cairo_surface_t *surface,
const cairo_surface_backend_t *backend,
cairo_device_t *device,
cairo_content_t content,
cairo_bool_t is_vector);
This has been broken since b1192bea (2015!)
(snuck additional compiler warning commit into this MR as well, but can split it out of course)