fftools/ffmpeg: track a list of non-lavfi outputs in InputStream

Currently, output streams where an input stream is sent directly (i.e.
not through lavfi) are determined by iterating over ALL the output
streams and skipping the irrelevant ones. This is awkward and
inefficient.

fftools/ffmpeg_mux_init.c

@@ -647,6 +647,9 @@ static OutputStream *new_output_stream(Muxer *mux, const OptionsContext *o,
     if (ost->ist) {
         ost->ist->discard = 0;
         ost->ist->st->discard = ost->ist->user_set_discard;
+
+        if (!(ost->enc && (type == AVMEDIA_TYPE_VIDEO || type == AVMEDIA_TYPE_AUDIO)))
+            ist_output_add(ost->ist, ost);
     }
     ost->last_mux_dts = AV_NOPTS_VALUE;