how much the line advances after drawing this glyph when setting text in horizontal direction.
how much the glyph moves on the X-axis before drawing it, this should not affect how much the line advances.
how much the line advances after drawing this glyph when setting text in vertical direction.
how much the glyph moves on the Y-axis before drawing it, this should not affect how much the line advances.
The #hb_glyph_position_t is the structure that holds the positions of the glyph in both horizontal and vertical directions. All positions in #hb_glyph_position_t are relative to the current point.