prepare sharing C++ class instances across lua-interpreters

in particular: lua-lifefime (!) C++ instances.
This allows for dynamic allocation of custom user-data, bound to
the lifetime of the allocating lua-context.

libs/lua/LuaBridge/detail/LuaRef.h

@@ -180,6 +180,11 @@ private:
       return *this;
     }
 
+    // the implementation needs UserdataPtr, which
+    // is not yet defined here.
+    // -> libs/ardour/lua_api.cc
+    Proxy& clone_instance (const void* key, void* p);
+
     //--------------------------------------------------------------------------
     /**
         Assign a new value to this table key.

libs/lua/LuaBridge/detail/Userdata.h

@@ -302,6 +302,11 @@ ud __parent (nil)
 public:
   virtual ~Userdata () { }
 
+  static void* get_ptr (lua_State* L, int index) {
+    Userdata* ud = static_cast <Userdata*> (lua_touserdata (L, index));
+    return ud->m_p;
+  }
+
   //--------------------------------------------------------------------------
   /**
     Returns the Userdata* if the class on the Lua stack matches.
@@ -457,6 +462,15 @@ private:
     assert (m_p != 0);
   }
 
+  friend class LuaRef;
+  static inline void push_raw (lua_State* const L, void* p, const void* classkey)
+  {
+    new (lua_newuserdata (L, sizeof (UserdataPtr))) UserdataPtr (p);
+    lua_rawgetp (L, LUA_REGISTRYINDEX, classkey);
+    assert (lua_istable (L, -1));
+    lua_setmetatable (L, -2);
+  }
+
 public:
   /** Push non-const pointer to object.
   */