[luatex] Interfere with page breaking to balance pages

[Patrick Gundlach]

> For me the following works
> 
> \directlua{
>  function myfunc(box)
>    texio.write_nl('box = ' .. box)
>  end
> }
> 
> \AtBeginShipout{%
>  \directlua{myfunc(\the\AtBeginShipoutBox)}%
> }

you can do something like this:

tex.box["AtBeginShipoutBox"]

that's a bit more luaeque IMO.

Patrick