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.h

@@ -202,6 +202,7 @@ typedef struct OutputFilter {
 
     /* temporary storage until stream maps are processed */
     AVFilterInOut       *out_tmp;
+    enum AVMediaType     type;
 } OutputFilter;
 
 typedef struct FilterGraph {
@@ -424,6 +425,7 @@ int configure_filtergraph(FilterGraph *fg);
 int configure_output_filter(FilterGraph *fg, OutputFilter *ofilter, AVFilterInOut *out);
 int ist_in_filtergraph(FilterGraph *fg, InputStream *ist);
 FilterGraph *init_simple_filtergraph(InputStream *ist, OutputStream *ost);
+int init_complex_filtergraph(FilterGraph *fg);
 
 int avconv_parse_options(int argc, char **argv);