[HarfBuzz] Rendering of Arabic shadda-kasrah

[Eli Zaretskii]

Could someone please look at the discussion and the data of the Emacs
bug#34035 (https://debbugs.gnu.org/cgi/bugreport.cgi?bug=34035) and
tell whether the fonts that produce incorrect display are faulty, and
if so, what is the problem with those fonts?  Also, is there perhaps
some way around these problems that would yield better results even
with the fonts which currently display the kasrah below the base
letter?  Because I tried many different fonts with reasonable coverage
of Arabic, and the vast majority of them produce this problem, so it
seems like the fonts which don't are quite rare.

(As you see from the last messages, hb-view produces the same display
as Emacs with HarfBuzz, so at least we are doing no worse in these
cases, and we have reason to believe this is not an Emacs-specific
problem.)

TIA