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.

For run-time checking.