[ot] Hook up horizontal kerning to kern table

Seems to work.  Yay!

Still to do: run kerning if GPOS doesn't have 'kern' feature.

src/hb-ot-font.cc

@@ -665,7 +665,7 @@ retry:
     hb_font_funcs_set_glyph_v_advance_func (funcs, hb_ot_get_glyph_v_advance, nullptr, nullptr);
     //hb_font_funcs_set_glyph_h_origin_func (funcs, hb_ot_get_glyph_h_origin, nullptr, nullptr);
     //hb_font_funcs_set_glyph_v_origin_func (funcs, hb_ot_get_glyph_v_origin, nullptr, nullptr);
-    //hb_font_funcs_set_glyph_h_kerning_func (funcs, hb_ot_get_glyph_h_kerning, nullptr, nullptr); TODO
+    hb_font_funcs_set_glyph_h_kerning_func (funcs, hb_ot_get_glyph_h_kerning, nullptr, nullptr);
     //hb_font_funcs_set_glyph_v_kerning_func (funcs, hb_ot_get_glyph_v_kerning, nullptr, nullptr);
     hb_font_funcs_set_glyph_extents_func (funcs, hb_ot_get_glyph_extents, nullptr, nullptr);
     //hb_font_funcs_set_glyph_contour_point_func (funcs, hb_ot_get_glyph_contour_point, nullptr, nullptr); TODO