lavfi/avfilter: export a multikey dict from process_options()

This way the function does not need to be able to match keys to
AVOptions, which will be useful in future commits.

libavfilter/avfilter.c

@@ -850,17 +850,7 @@ static int process_options(AVFilterContext *ctx, AVDictionary **options,
                 return ret;
             }
         } else {
-            o = av_opt_find(ctx->priv, key, NULL, 0,
-                            AV_OPT_SEARCH_CHILDREN | AV_OPT_SEARCH_FAKE_OBJ);
-            if (!o) {
-                av_log(ctx, AV_LOG_ERROR, "Option '%s' not found\n", key);
-                av_free(value);
-                av_free(parsed_key);
-                return AVERROR_OPTION_NOT_FOUND;
-            }
-            av_dict_set(options, key, value,
-                        (o->type == AV_OPT_TYPE_FLAGS &&
-                         (value[0] == '-' || value[0] == '+')) ? AV_DICT_APPEND : 0);
+            av_dict_set(options, key, value, AV_DICT_MULTIKEY);
         }
 
         av_free(value);