@behdad thanks for the very quick response to #83 (closed)! I believe I've implemented it in the best way possible, deprecating the four old functions,
pango_(ft2|xft)_font_map_(set_default_substitute|substitute_changed), adding the new FC
substitute_changed functions, and hooking up the new symbols.
The user-supplied callback is invoked by the child classes in the exact same fashion as before, and PangoCairoFontMap does this now too. The callback is stored on the parent class's (PangoFcFontMap) instance.
Cache clearing happens in the parent class, and I'm calling
PangoFontMap->changed to increment the serial. Both of those clean up some code. Should be functionally identical to before.
Fixes #83 (closed)