Avoid layout work for boxes that end up not generated.
Avoid layout work for boxes that end up not generated.

marclaporte/WeasyPrint,Kozea/WeasyPrint,jasco/WeasyPrint,marclaporte/WeasyPrint,Kozea/WeasyPrint,prepare/TestWeasyPrint,andrewleech/WeasyPrint,prepare/TestWeasyPrint,jasco/WeasyPrint,andrewleech/WeasyPrint