fix most freetype/harfbuzz related memory leaks

[benoit-pierre]

There's still one small leak remaining due to using FT_Done_Library instead of FT_Done_FreeType to handle the Freetype's library lifecycle: its memory object is not freed on exit.

This is also the precursor to some memory optimizations.

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This change is 

[poire-z]

fix most freetype/harfbuzz related memory leaks

Btw, how much of that amount to the well known KOReader memory leaks (cf koreader/koreader#5998 (comment)) ?
Does your mesonic monolibtic build still get its memory usage increasing as obviously as ours ?

[poire-z]

A bit over my head, so it must be ok :)

[benoit-pierre]

Good question, memory does increase as more features get used, menus opened, but I'm not sure it's leaking. Honestly, I don't have to restart on my Kindle anymore thanks to:

• those changes coupled with the aforementioned fonts related memory optimizations: it shaves 30 MB from Koreader's memory use after browsing in the file manager, opening a (previously opened) EPUB, and triggering the various bottom settings panels
• using my meson' size optimized build (buildtype = 'minsize'), combined with a pass on all libraries to ensure GNU visibility support is used, as well as trimming unnecessary features and statically linking when applicable: it shaves about 10 MB from the combined BSS+data+code sections

monolibtic is currently broken (because of the visibility patches), but technically fixable.

[NiLuJe]

I'm not up to snuff on the ft/hb API, but this looks sound ;).