[Fontconfig] SFNT TTF support in fontconfig

[Qianqian Fang]

hi

I noticed that the fc-cache utility does not support SFNT ttf(or otf) 
format well. It will output a fonts.cache-1 file with wrong font information.

The specific sfnt font I tested is WenQuanYi's bitmap CJK font,
which is generated with fontforge, and can be download here
http://prdownloads.sourceforge.net/wqy/wqy-bitmapfont-ttf-0.7.0-4.tar.gz?download

If I create fonts.cache-1 manually with hand-written font information,
"fc-cache -f" will overwrite it with erroneous info.

I found a couple of Sfnt calls from fcfreetype.c unit, seems it is
designed to support this format. I am wondering if anyone can 
give me some insight on what might cause the failure of recognition 
of wenquanyi's font.

meanwhile, in fc-cache.c, I don't know if it possible to add
a skipping flag under "-f" option, for example, if fc-cache finds
a file named fonts.cache-forceskip under the current directory, it will 
skip this font folder under any condition. I think this might be
helpful if font developers want to put some specific settings and
don't want system to overwrite.

thanks

Qianqian