This is a prerequisite to implement hyperlinks in PDF output (bug 454059), because the current version of cairo we have in the tree doesn't include the necessary APIs (cairo_tag_begin, cairo_tag_end) to create hyperlinks.

The latest Cairo snapshot (currently 1.17.4) provides this, although we will need to merge some of our local modifications in order to integrate it with gecko; it's not usable just as a drop-in replacement.