Fix previous commit

Priority should be given to specific over dispatch.  Broke sanitize before.
This fixes it, by moving prioritization to the context implementation, since
the correct priority cannot be done in the dispatch implementation.  Done
for subset and sanitize only, which need it.

src/hb-sanitize.hh

diff --git a/src/hb-sanitize.hh b/src/hb-sanitize.hh
index 71bf3a8..6799b78 100644
--- a/src/hb-sanitize.hh
+++ b/src/hb-sanitize.hh
@@ -130,13 +130,23 @@
   template <typename T, typename F>
   bool may_dispatch (const T *obj HB_UNUSED, const F *format)
   { return format->sanitize (this); }
-  template <typename T, typename ...Ts>
-  return_t _dispatch (const T &obj, Ts &&...ds)
-  { return obj.sanitize (this, hb_forward<Ts> (ds)...); }
   static return_t default_return_value () { return true; }
   static return_t no_dispatch_return_value () { return false; }
   bool stop_sublookup_iteration (const return_t r) const { return !r; }
 
+  private:
+  template <typename T, typename ...Ts> auto
+  _dispatch (const T &obj, hb_priority<1>, Ts &&...ds) HB_AUTO_RETURN
+  ( obj.sanitize (this, hb_forward<Ts> (ds)...) )
+  template <typename T, typename ...Ts> auto
+  _dispatch (const T &obj, hb_priority<0>, Ts &&...ds) HB_AUTO_RETURN
+  ( obj.dispatch (this, hb_forward<Ts> (ds)...) )
+  public:
+  template <typename T, typename ...Ts> auto
+  dispatch (const T &obj, Ts &&...ds) HB_AUTO_RETURN
+  ( _dispatch (obj, hb_prioritize, hb_forward<Ts> (ds)...) )
+
+
   void init (hb_blob_t *b)
   {
     this->blob = hb_blob_reference (b);