[cplusplus] Internally allow using hb_unique_ptr with hb_free()

...for arbitrary types.

src/hb.hh

@@ -466,10 +466,23 @@ static_assert ((sizeof (hb_mask_t) == 4), "");
 static_assert ((sizeof (hb_var_int_t) == 4), "");
 
 
+#include "hb-cplusplus.hh"
+
+namespace hb {
+
+// Allow using hb::unique_ptr with hb_free() internally.
+template <typename T>
+struct vtable
+{
+  static constexpr auto destroy = hb_free;
+};
+
+}
+
+
 /* Headers we include for everyone.  Keep topologically sorted by dependency.
  * They express dependency amongst themselves, but no other file should include
  * them directly.*/
-#include "hb-cplusplus.hh"
 #include "hb-meta.hh"
 #include "hb-mutex.hh"
 #include "hb-number.hh"