fftools/ffmpeg: open decoders right after we know they are needed

Will allow initializing subtitle encoding earlier.

fftools/ffmpeg.c

@@ -1882,11 +1882,6 @@ static int transcode_init(void)
                 ifile->streams[j]->start = av_gettime_relative();
     }
 
-    /* init input streams */
-    for (InputStream *ist = ist_iter(NULL); ist; ist = ist_iter(ist))
-        if (ist->decoding_needed && (ret = dec_open(ist)) < 0)
-            goto dump_format;
-
     /*
      * initialize stream copy and subtitle/data streams.
      * Encoded AVFrame based streams will get initialized when the first AVFrame

fftools/ffmpeg_demux.c

@@ -566,6 +566,12 @@ static void ist_use(InputStream *ist, int decoding_needed)
     ist->discard          = 0;
     ist->st->discard      = ist->user_set_discard;
     ist->decoding_needed |= decoding_needed;
+
+    if (decoding_needed && !avcodec_is_open(ist->dec_ctx)) {
+        int ret = dec_open(ist);
+        if (ret < 0)
+            report_and_exit(ret);
+    }
 }
 
 void ist_output_add(InputStream *ist, OutputStream *ost)