Add a dummy indirection in C++ for the item vtable access

This will allow for a compile-time decision in the future whether
to use the vtable symbol or getter function.

api/sixtyfps-cpp/include/sixtyfps.h

@@ -204,6 +204,12 @@ inline vtable::Layout drop_in_place(ComponentRef component)
     return vtable::Layout { sizeof(T), alignof(T) };
 }
 
+template<typename VTableType>
+constexpr inline const VTableType *get_vtable(const VTableType *vtable_symbol)
+{
+    return vtable_symbol;
+}
+
 template<typename T>
 struct ReturnWrapper
 {