Submitted by Behdad Esfahbod
We have default substitute funcs for ft2 and xft backends, but not cairo. I know the reasoning, but I still think that it comes handy and useful. For example I can imagine this use for it: firefox has a font preferences page where user sets preferred font per script. Now to make pango respect that you either have to break on scripts (using PangoScriptIter) first and call pango_itemize() with different font descriptions for each script segment, or you could just set a fc default substitute function to handle it...