[HarfBuzz] How to get a glyph code for a character?

[Eli Zaretskii]

One of the methods an Emacs font-backend should provide is the
encode_char method, which returns the glyph code of the selected font
for a character given by its Unicode codepoint.  For example, the XFT
backend uses the XftCharIndex function for that purpose, and the
Freetype backend uses FT_Get_Char_Index.

What is the best way of providing such a method with HarfBuzz on
MS-Windows?  One possibility is obviously to call hb_shape, but maybe
there's a simpler way for a single codepoint?

Btw, what does hb_font_get_glyph() return?

TIA