avconv: split creating and (re-)configuring complex filtergraphs

The current code is less than straightforward due to the fact that
output streams can be created based on filtergraph definitions. This
change should make the code simpler and more readable. It will also be
useful in the future commits.

avconv.c

@@ -1679,11 +1679,6 @@ static int transcode_init(void)
                 input_streams[j + ifile->ist_index]->start = av_gettime_relative();
     }
 
-    /* init complex filtergraphs */
-    for (i = 0; i < nb_filtergraphs; i++)
-        if ((ret = avfilter_graph_config(filtergraphs[i]->graph, NULL)) < 0)
-            return ret;
-
     /* for each output stream, we compute the right encoding parameters */
     for (i = 0; i < nb_output_streams; i++) {
         AVCodecContext *enc_ctx;