(font_otf_capability): Fix handling of the default

(font_otf_capability): Fix handling of the default
langsys.
(parse_gsub_gpos_spec): Type changed to void.  New arg nbytes.
Check the contents of SPEC.
(LGSTRING_HEADER_SIZE, LGSTRING_GLYPH_SIZE): New macros.
(check_gstring): New function.
(REPLACEMENT_CHARACTER): New macro.
(font_otf_gsub): New arg alternate_subst.  Be sure to set all
glyph codes of GSTRING.
(font_otf_gpos): Be sure to set all glyph codes of GSTRING.
(font_prepare_composition): Set cmp->glyph_len.
(font_open_entity): Set font->scalable.
(Ffont_get): Handle :otf property.
(Ffont_otf_gsub, Ffont_otf_gpos, Ffont_otf_alternates): New
functions.
(Fquery_font): Use font->font.full_name.
(syms_of_font): Defsubr Sfont_otf_gsub, Sfont_otf_gpos, and
Sfont_otf_alternates.