* hb_aat_get_feature_settings

Ouch!

A starting point, its sanitization is not tested however

I think it makes sense to accumulate it, but Ned tells me that's
what CoreText does.

Finally:  Fixes https://github.com/harfbuzz/harfbuzz/issues/1356

Test case:
$ ./hb-shape GeezaPro.ttc -u U+0628,U+064A,U+064E,U+0651,U+0629
[u0629.final.tehMarbuta=4+713|u064e_u0651.shaddaFatha=1@0,-200+0|u064a.medial.yeh=1+656|u0628.initial.beh=0+656]

The mark positioning (kern table CrossStream kerning) only works if deleted
glyph (as result of ligation) is still in stream and pushed through the
state machine.

mort really needs some initial tests at least.

Yoohoo!

https://github.com/harfbuzz/harfbuzz/issues/1342#issuecomment-434829028

Untested.

Started by reverting https://github.com/harfbuzz/harfbuzz/commit/1f1c85a5

Just a starting point, if we agree even mort can come back.

Only works at entire-buffer range, not sub-ranges.

Test with:

$ hb-shape Zapfino.dfont Zapfino
[Z_a_p_f_i_n_o=0+2333]

$ hb-shape Zapfino.dfont Zapfino --features=-dlig
[Z=0+416|a=1@-21,0+264|p_f=2+433|i=4+181|n=5+261|o=6+250]

$ hb-shape Zapfino.dfont Zapfino --features=+dlig
[Z_a_p_f_i_n_o=0+2333]

Fixes https://github.com/harfbuzz/harfbuzz/issues/1303

Need to map enum values to numerics since we don't have CoreText headers.

Works beautifully!  Test coming.

Tested with Kannada MN:

$ HB_OPTIONS=aat ./hb-shape Kannada\ MN.ttc -u 0CCD,0C95,0CD6
[kn_ka.vattu=0+230|kn_ai_length_mark=1@326,0+607]

Not hooked up to be called yet.

Sorry for the noise, downstream custom builders.  Please adjust.

We are not going to use them, at least in the foreseeable future

Also, uncomment code again, just "if (0)" it out, so it doesn't get stale again.

We should hang those off somewhere else. For now, the unused ones can go.