the index of the character in the original text that corresponds
to this #hb_glyph_info_t, or whatever the client passes to
hb_buffer_add(). More than one #hb_glyph_info_t can have the same
cluster value, if they resulted from the same character (e.g. one
to many glyph substitution), and when more than one character gets
merged in the same glyph (e.g. many to one glyph substitution) the
#hb_glyph_info_t will have the smallest cluster value of them.
By default some characters are merged into the same cluster
(e.g. combining marks have the same cluster as their bases)
even if they are separate glyphs, hb_buffer_set_cluster_level()
allow selecting more fine-grained cluster handling.
either a Unicode code point (before shaping) or a glyph index (after shaping).
The #hb_glyph_info_t is the structure that holds information about the glyphs and their relation to input text.