gtkprintjob: release the surface and backend before the output io channel

gtkprintjob: release the surface and backend before the output io channel

If the GtkPrintJob is freed too early when the surface has been created
but the job hasn't been sent to the printer, it's possible that the
file print backend tries to write to the io chaneel when it is already
closed. This produces runtime critical warnings:

GLib-CRITICAL **: g_io_channel_write_chars: assertion `channel->is_writeable' failed

https://bugzilla.gnome.org/show_bug.cgi?id=685420