(struct ftfont_info): New member index, delete member

(struct ftfont_info): New member index, delete member
fc_charset_idx.  Make the member order compatible with struct
xftfont_info.
(fc_charset_table): Change charset names to registry names.
(ftfont_pattern_entity): Delete the args registry and
fc_charset_idx.  Change the value of :font-entity property
to (FONTNAME . INDEX).  Always set :registry property to
`iso10646-1'.
(struct ftfont_cache_data): New struct.
(ftfont_lookup_cache): New arg for_face.
(ftfont_get_fc_charset, ftfont_get_otf): New functions.
(ftfont_driver): Set the member otf_capability.
(ftfont_get_charset): Adjust it for the change of
fc_charset_table.
(OTF_TAG_SYM): New macro.
(ftfont_spec_pattern): Delete the arg fc_charset_idx.  Adjust it
for the change of fc_charset_table.
(ftfont_list): Adjust it for the change of ftfont_spec_pattern and
ftfont_pattern_entity.  Add FC_INDEX to objset.
(ftfont_match): Adjust it for the change of ftfont_spec_pattern
and ftfont_pattern_entity.
(ftfont_open): Adjust it for the change of ftfont_lookup_cache,
font_make_object, struct ftfont_info.
(ftfont_has_char): Use ftfont_get_fc_charset.
(ftfont_otf_features, ftfont_otf_capability): New functions.
(ftfont_shape): Use ftfont_get_otf.