gui: Fix printing of tables split across pages in psppire-output-window.

xr_driver_output_item() expects the caller to call xr_driver_next_page()
as long as xr_driver_need_new_page() returns true, but the printing code
didn't do that, so tables split across pages in printouts caused an
assertion failure at the next call to xr_driver_output_item().

Reported-by: John Darrington <john@darrington.wattle.id.au>