Sort anchors across pages
Sort anchors across pages

Previously we had sorted anchors within one page, but if anchors happened to be
out of order *across* pages, they wouldn't be written in order, which caused
lingering issues in some viewers.

Related to #1352 and the change in d4561b13af89e89acb66586b1e2da66dfaba1fac.

Kozea/WeasyPrint,Kozea/WeasyPrint