[trak] Fix, and hook up

Works beautifully!  Test coming.

src/hb-aat-layout.cc

diff --git a/src/hb-aat-layout.cc b/src/hb-aat-layout.cc
index 73ab06d..2b86ba8 100644
--- a/src/hb-aat-layout.cc
+++ b/src/hb-aat-layout.cc
@@ -37,7 +37,7 @@
 #include "hb-aat-ltag-table.hh" // Just so we compile it; unused otherwise.
 
 /*
- * morx/kerx/trak/ankr
+ * morx/kerx/trak
  */
 
 static inline const AAT::morx&
@@ -82,6 +82,12 @@
     *blob = hb_ot_face_data (face)->ankr.get_blob ();
   return ankr;
 }
+static inline const AAT::trak&
+_get_trak (hb_face_t *face)
+{
+  if (unlikely (!hb_ot_shaper_face_data_ensure (face))) return Null(AAT::trak);
+  return *(hb_ot_face_data (face)->trak.get ());
+}
 
 
 hb_bool_t
@@ -124,3 +130,20 @@
 				 ankr, ankr_blob->data + ankr_blob->length);
   kerx.apply (&c);
 }
+
+hb_bool_t
+hb_aat_layout_has_tracking (hb_face_t *face)
+{
+  return _get_trak (face).has_data ();
+}
+
+void
+hb_aat_layout_track (hb_ot_shape_plan_t *plan,
+		     hb_font_t *font,
+		     hb_buffer_t *buffer)
+{
+  const AAT::trak& trak = _get_trak (font->face);
+
+  AAT::hb_aat_apply_context_t c (plan, font, buffer);
+  trak.apply (&c);
+}