Consider caching charset subset relation
I have code here that computes the FcCharSetIsSubset relations for all fonts in the current configuration, saves that data to a file, and loads it again:
https://gitlab.gnome.org/GNOME/pango/-/merge_requests/229
That makes coverage trimming much cheaper, since most of the charset comparisons can be avoided. The code probably belongs into fontconfig, but I don't have the energy to rewrite it and figure out how to integrate it into fc-cache.
It is slighty tricky, since it is global information, not per-directory.
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