Minor.
[autofit] Handle `DFLT' OpenType script for coverages.

* include/ftautoh.h: Document new `default-script' property.

* src/autofit/hbshim.c (af_get_coverage): Use `AF_FaceGlobals' for
type of first parameter.
(script_tags): Add one more element.
(af_get_coverage): Adjust `script_tags' to handle `DFLT' script tag.

* src/autofit/hbshim.h: Updated.

* src/autofit/afglobal.c (af_face_globals_compute_style_coverage):
Updated.

* src/autofit/afglobal.h (AF_SCRIPT_DEFAULT): New macro.

* src/autofit/afmodule.h (AF_ModuleRec): New `default_script'
member.

* src/autofit/afmodule.c (af_property_set, af_property_get): Handle
`default-script' property.
(af_autofitter_init): Updated.
[autofit] Use `global' HarfBuzz font object.

We now use `hb_font' instead of `hb_face' since yet-to-come changes
need this.

* src/autofit/afglobal.h: Include `hbshim.h'.
(AF_FaceGlobalsRec) [FT_CONFIG_OPTION_USE_HARFBUZZ]: New member
`hb_font'.

* src/autofit/afglobal.c (af_face_globals_new)
[FT_CONFIG_OPTION_USE_HARFBUZZ]: Create `hb_font'.
(af_face_globals_free) [FT_CONFIG_OPTION_USE_HARFBUZZ]: Destroy
`hb_font'.

* src/autofit/hbshim.h: Include HarfBuzz headers.

* src/autofit/hbshim.c: Include `hbshim.h' instead of HarfBuzz
headers.
(af_get_coverage): Updated.
[autofit] Add wrapper function for `FT_Get_Char_Index'.

Yet-to-come changes will provide HarfBuzz functionality for the new
function.

* src/autofit/hbshim.c (af_get_char_index): New function.
* src/autofit/hbshim.h: Updated.

* src/autofit/afcjk.c (af_cjk_metrics_init_widths,
af_cjk_metrics_init_blues, af_cjk_metrics_check_digits): Updated.

* src/autofit/aflatin.c (af_latin_metrics_init_widths,
af_latin_metrics_init_blues, af_latin_metrics_check_digits):
Updated.
[autofit] Code shuffling to reduce use of cpp macros.

* src/autofit/afglobal.c (af_face_globals_compute_style_coverage):
Call `af_get_coverage' unconditionally.

* src/autofit/autofit.c: Include `hbshim.c' unconditionally.

* src/autofit/hbshim.c (af_get_coverage)
[!FT_CONFIG_OPTION_USE_HARFBUZZ]: Provide dummy function.

* src/autofit/hbshim.h: Provide function declarations
unconditionally.