[autofit] Don't combine multiple features into one set.

Combining them, as originally envisioned, would lead to much more
complicated code, as investigations have shown meanwhile.  The major
drawback is that we run out of available style slots much earlier.
However, this is only a theoretical issue since we don't support a
large number of scripts currently.

* src/autofit/afcover.h: Replace `COVERAGE_{1,2,3}' macros with
a single-element `COVERAGE' macro, sort the elements by the feature
tags, and add entry for `ruby'.

* src/autofit/aftypes.h: Updated.
* src/autofit/hbshim.c: Updated.
[autofit] Fix interface of `af_get_char_index'.

* src/autofit/hbshim.c (af_get_char_index): Return error value.
Add argument for y offset (to be used in a yet-to-come patch).

* src/autofit/hbshim.h, src/autofit/afcjk.c,
src/autofit/aflatin.c: Updated.
[autofit] s/AF_STYLE_...._DEFAULT/AF_STYLE_...._DFLT/i.
[autofit] Implement and use `af_get_char_index' with HarfBuzz.

* src/autofit/hbshim.c (COVERAGE) [FT_CONFIG_OPTION_USE_HARFBUZZ]:
Redefine to construct HarfBuzz features.
(af_get_char_index) [FT_CONFIG_OPTION_USE_HARFBUZZ]: Rewritten.

* src/autofit/aflatin.c (af_latin_metrics_init_blues): Use
`y_offset' to adjust `best_y'.
* src/autofit/hbshim.c: s/{lookups,glyphs}/gsub_{lookups,glyphs}/.
[autofit] Fix style assignments to glyphs.

* src/autofit/hbshim.c (af_get_coverage)
[FT_CONFIG_OPTION_USE_HARFBUZZ]: Scan GPOS coverage of features also
so that we can skip glyphs that have both GSUB and GPOS data.
[autofit] Add more styles.

* src/autofit/afstyles.h (STYLE_LATIN, META_STYLE_LATIN): New
auxiliary macros; use them to define styles for Cyrillic, Greek, and
Latin.

* src/autofit/afcover.h: Remove coverage for oldstyle figures.
Since those digits are used in combination with ordinary letters, it
makes no sense to handle them separately.

* src/autofit/afglobal.c (af_face_globals_get_metrics): Don't limit
`options' parameter to 4 bits.
[autofit] Remove some styles.

* src/autofit/afcover.h: Remove coverages for alternative fractions,
denominators, numerators, and fractions.

* src/autofit/afstyles.h (META_STYLE_LATIN): Updated.
[autofit] Better ftgrid support.

* src/autofit/afhints.c (af_glyph_hints_get_segment_offset): Add
parameters `is_blue' and `blue_offset'.