#hb_font_t to work upon
The glyph ID of the first glyph in the glyph pair to query
The glyph ID of the second glyph in the glyph pair to query
The direction of the text segment
The horizontal kerning-adjustment value retrieved
The vertical kerning-adjustment value retrieved
Fetches the kerning-adjustment value for a glyph-pair in the specified font.
Calls the appropriate direction-specific variant (horizontal or vertical) depending on the value of direction.