(font_parse_fcname): Don't change :name property of FONT.

(font_parse_fcname): Don't change :name property of FONT.
(font_parse_name): Likewise.
(LGSTRING_HEADER_SIZE, LGSTRING_GLYPH_SIZE, check_gstring): Define
them unconditionally.
(font_matching_entity): New function.
(font_open_by_name): Try font_matching_entity if exact match is
not found.
(font_update_drivers): Delete the arg FONT.  Return a list of
actually used backends.  Don't free faces, font caches here.
Don't store data in frame parameters.  Don't call x_set_font.
(Ffont_spec): Store :name property as is.
(Ffont_get): Check HAVE_LIBOTF before calling font_otf_capability.
(Ffont_otf_gsub): Call font->driver->otf_gsub instead of
font_otf_gsub.
(Ffont_otf_gpos): Call font->driver->otf_gpos instead of
font_otf_gpos.
(Ffont_otf_alternates): Check if the driver has otf_gsub function.
Call font->driver->otf_gsub instead of font_otf_gsub.