[Fontconfig] Another proposal for saving custom fonts in fontconfig

[Peng Wu]

Hi,

  Based on suggestions from Akira TAGOH, I propose another API
for this purpose.

  Take an example which we plan to override "fontfeatures" and
"fontfeatures" properties:

customfont = FcDirCacheBeginCustomFont(config, file, index, stylename);
  FcCustomFontModifyString(customfont, FC_FONT_FEATURES, features);
  FcCustomFontModifyString(customfont, FC_FONT_VARIATIONS, variations);
FcDirCacheEndCustomFont(customfont);

Thanks,
  Peng