[sfnt] Synthesize a Unicode charmap if one is missing.
* src/sfnt/ttcmap.h (tt_cmap_unicode_class_rec): Declare it.
* src/sfnt/ttcmap.c (tt_get_glyph_name, tt_cmap_unicode_init,
tt_cmap_unicode_done, tt_cmap_unicode_char_index,
tt_cmap_unicode_char_next, tt_cmap_unicode_class_rec): Implement
synthetic Unicode charmap class.
(tt_get_cmap_info): Make sure the callback is available.
* src/sfnt/sfobjs.c (sfnt_load_face)
[FT_CONFIG_OPTION_POSTSCRIPT_NAMES]: If Unicode charmap is missing,
synthesize one.
* include/freetype/config/ftoption.h: Document it.
* devel/ftoption.h: Ditto.
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