Force grayscale font AA with Cairo on Windows (bug #21648)

Cairo on Windows (at least the latest available version from gtk.org
as of 2014-02-22, version 1.10.2) has issues with ClearType resulting
in glitchy anti-aliasing with CAIRO_ANTIALIAS_SUBPIXEL or
CAIRO_ANTIALIAS_DEFAULT, but not CAIRO_ANTIALIAS_GRAY, so we should
use that as a workaround until the Windows package is updated to use
a newer version of Cairo.
Merge branch 'master' of github.com:wesnoth/wesnoth-old