[coretext] Fully support user features

Based on patch from Jonathan Kew and data from Apple.

It's not working correctly though, and I suspect I'm hitting a bug in
CoreText.  When I do this:

hb-shape /Library/Fonts/Zapfino.ttf ZapfinoZapfino --shaper coretext \
--features=-liga

I expect both ligatures to turn off, but only the second one does:

[Z_a_p_f_i_n_o=0+2333|Z=7+395|a=8+285|p_f=9+433|i=11+181|n=12+261|o=13+250]

whereas if I disable 'dlig' instead of 'liga', both are turned off.
Smells...

Doesn't resolve conflicting feature settings.
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