Implement the otf_capability method for HarfBuzz

Implement the otf_capability method for HarfBuzz

* src/hbfont.c: Include hb-ot.h.
[HAVE_NTGUI]: Add DEF_DLL_FN and #define for hb_tag_to_string,
hb_font_get_face, hb_ot_layout_table_get_script_tags,
hb_ot_layout_table_get_feature_tags, hb_ot_layout_script_get_language_tags,
and hb_ot_layout_language_get_feature_tags.
(hbfont_init_w32_funcs) [HAVE_NTGUI]: Add LOAD_DLL_FN for them.
(hbfont_otf_features, hbfont_otf_capability): New functions.

* src/font.h (hbfont_otf_capability) [HAVE_HARFBUZZ]: Add extern.

* src/ftcrfont.c (syms_of_ftcrfont_for_pdumper) [HAVE_HARFBUZZ]:
* src/ftfont.c (syms_of_ftfont_for_pdumper) [HAVE_HARFBUZZ]:
* src/w32uniscribe.c (syms_of_w32uniscribe_for_pdumper) [HAVE_HARFBUZZ]:
* src/xftfont.c (syms_of_xftfont_for_pdumper) [HAVE_HARFBUZZ]: Populate
otf_capability method with hbfont_otf_capability.