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.h

@@ -416,6 +416,14 @@ typedef struct InputStream {
     InputFilter **filters;
     int        nb_filters;
 
+    /*
+     * Output targets that do not go through lavfi, i.e. subtitles or
+     * streamcopy. Those two cases are distinguished by the OutputStream
+     * having an encoder or not.
+     */
+    struct OutputStream **outputs;
+    int                nb_outputs;
+
     int reinit_filters;
 
     /* hwaccel options */
@@ -867,6 +875,8 @@ void ifile_close(InputFile **f);
  */
 int ifile_get_packet(InputFile *f, AVPacket **pkt);
 
+void ist_output_add(InputStream *ist, OutputStream *ost);
+
 /* iterate over all input streams in all input files;
  * pass NULL to start iteration */
 InputStream *ist_iter(InputStream *prev);