[cairo] Text: Subpixel positioning of glyphs

[Nikolaus Waxweiler]

Hi again,
I'm interested in Gtk gaining subpixel positioning of glyphs. Matthias
Claasen has done some work here:
https://gitlab.gnome.org/GNOME/gtk/issues/7#note_301901

This commit:
https://github.com/matthiasclasen/cairo/commit/052281c695f206f253cb9805543a0b18ba647c3c
adds subpixel positioning to the image backend and it seems to work
judging from test suite images, aside from some failures due to
misaligned text (e.g. text-rotate seems to be slightly broken by the
commit?).

Test suite failures aside, are there objections or general gotchas for
that patch? I still have to figure out what else is to be done to enable
this in Gtk.