transcode: forward the sout_stream_t object

The object is needed to forward the elementary streams and data to the
sout stream components following the transcode.

Note that having the stream object here means that we must ensure we
don't call it after the transcode is supposed to be closed, so we must
ensure every asynchronous components are drained and destroyed before
leaving the close functions.

The constraint was already there though, given p_obj actually pointed to
the p_stream object too.

modules/stream_out/transcode/transcode.c

@@ -579,7 +579,7 @@ static void *Add( sout_stream_t *p_stream, const es_format_t *p_fmt )
     struct decoder_owner * p_owner = vlc_object_create( p_stream, sizeof( *p_owner ) );
     if( !p_owner )
         goto error;
-    p_owner->p_obj = VLC_OBJECT(p_stream);
+    p_owner->p_stream = p_stream;
 
     id->p_decoder = &p_owner->dec;
     decoder_Init( id->p_decoder, p_fmt );

modules/stream_out/transcode/transcode.h

@@ -150,7 +150,7 @@ struct sout_stream_id_sys_t
 struct decoder_owner
 {
     decoder_t dec;
-    vlc_object_t *p_obj;
+    sout_stream_t *p_stream;
     sout_stream_id_sys_t *id;
 };