[Fontconfig] kerning improvements

[Keith Packard]

Around 6 o'clock on Sep 5, David Chester wrote:

> Basically, errors from rounding the advance widths are remembered and 
>  accounted for during layout, one level up (i.e. in Xft or ftstring, etc.).

Xft is designed as a relatively primitive API for placing glyphs; the 
higher level text layout functions are the likely candidates for adjusting 
the position of glyphs based on their unscaled metrics.  What Xft could 
do, however, is offer fixed-point widths computed from the unscaled 
metrics for applications to use in adjusting text as appropriate.

It's not really feasible for Xft to implement this internally as it isn't 
necessarily given the whole string to draw; results which differ when 
drawing a sub-string will generate visible artifacts on the screen.

The "right" place for this patch is in Qt, Pango, Mozilla, and other text 
layout systems.  Perhaps someday we can standardize on one layout 
mechanism, but there are still a wide range of ideas about how such a 
system should work, so it would be premature to pick one.

-keith