[Fontconfig] Greatly improved subpixel filtering in Xft

[Anders Kaseorg]

The current subpixel filtering in Xft is nearly useless, as black text 
tends to come out as a distracting mixture of red and blue:
   <http://www.kaseorg.com/~anders/subpixel-before.png>
That's because the filter is currently limited to staying within pixel 
boundaries. I wrote this patch to fix it:
   <http://www.kaseorg.com/~anders/libXft-improved-filtering.patch>
The result:
   <http://www.kaseorg.com/~anders/subpixel-after.png>

This is a *huge* improvement--even without proper gamma correction, I 
see almost no color fringing at all. It works for RGB, BGR, VRGB, and 
VBGR. The only side effect I've noticed is that the underlines for 
access keys in some GTK menus become a tiny bit wider (which is probably 
GTK's fault, since the menus in Mozilla applications are fine).

It currently uses a simple box filter, [1,1,1]/3, because that looked 
sharper to me than things like [1,3,4,3,1]/12, but the filter is trivial 
to change, and it can go outside the current pixel by arbitrary amounts.

Anders