GtkSettings: Consider default settings for font hinting and antialias

GtkSettings: Consider default settings for font hinting and antialias

Arrange things so that gtk-xdg-hinting==-1 and gtk-xfg-antialias==-1
end up as CAIRO_HINT_STYLE_DEFAULT and CAIRO_ANTIALIAS_DEFAULT in the
cairo font options.

This will not change anything on Linux desktops where xsettings will
always provide values different from -1. But on other platforms, we
can benefit from getting the platform-specific defaults in cairo.

Based on the first patch in:
https://bugzilla.gnome.org/show_bug.cgi?id=735316