[Fontconfig] Another proposal for saving custom fonts in fontconfig

[Peng Wu]

Hi Akira TAGOH,

  Here are the API requirement from pango side.

  Pango uses FcFontList to retrieve the complete list of fonts
available in the Operating System.

  And with variable fonts, there are some axis to define own style
custom fonts. The user may want save the custom font as new style
to be easy accessed next time.

  The current problem is that fontconfig can't return the custom font
list when querying with FcFontList.

  After several discussion, maybe we can save the custom font into 
fontconfig cache, and the custom font need to be preserved next time,
even after some fonts installed or removed if possible.

  With the custom font saved in fontconfig cache, the custom style font
will be listed in font chooser dialog. When use this custom style font
next time, pango will copy the saved font variations into
PangoFontDescription from fontconfig, and use the custom style font
as normal fonts except that the font variations is set.

  In future, pango may also copy OpenType features into
PangoFontDescription. Currently OpenType features seems implemented
by pango attributes.

  If you need more information about this API proposal,
please let me know.

Thanks,
  Peng