lavu/opt: use AV_DICT_MULTIKEY in av_opt_set_dict2()

If the dictionary provided on input contains multiple entries for an
option (relevant for flags modifying the previous value with '+' or
'-') and the option is not found in the target object, only the last
entry would be returned to the caller.

Pass AV_DICT_MULTIKEY to av_dict_set() to make sure all such entries are
returned.

libavutil/opt.c

@@ -1752,7 +1752,7 @@ int av_opt_set_dict2(void *obj, AVDictionary **options, int search_flags)
     while ((t = av_dict_iterate(*options, t))) {
         ret = av_opt_set(obj, t->key, t->value, search_flags);
         if (ret == AVERROR_OPTION_NOT_FOUND)
-            ret = av_dict_set(&tmp, t->key, t->value, 0);
+            ret = av_dict_set(&tmp, t->key, t->value, AV_DICT_MULTIKEY);
         if (ret < 0) {
             av_log(obj, AV_LOG_ERROR, "Error setting option %s to value %s.\n", t->key, t->value);
             av_dict_free(&tmp);