Given a `PangoGlyphItem` and the corresponding text, determine the width corresponding to each character.
When multiple characters compose a single cluster, the width of the entire cluster is divided equally among the characters.
See also [method@Pango.GlyphString.get_logical_widths].
text that this corresponds to (glyph_item->item->offset is an offset from the start of
an array whose length is the number of characters in glyph_item (equal to glyph_item->item->num_chars) to be filled in with the resulting character widths.