fftools/ffmpeg: refactor disabling decoder threading for attached pictures

* as this decision is based on demuxing information, move it from the
  decoder to the demuxer
* as the issue being addressed is latency added by frame threading, we
  only need to disable frame threading, not all threading

fftools/ffmpeg_dec.c

@@ -960,9 +960,6 @@ int dec_open(InputStream *ist, Scheduler *sch, unsigned sch_idx,
 
     if (!av_dict_get(*dec_opts, "threads", NULL, 0))
         av_dict_set(dec_opts, "threads", "auto", 0);
-    /* Attached pics are sparse, therefore we would not want to delay their decoding till EOF. */
-    if (ist->st->disposition & AV_DISPOSITION_ATTACHED_PIC)
-        av_dict_set(dec_opts, "threads", "1", 0);
 
     av_dict_set(dec_opts, "flags", "+copy_opaque", AV_DICT_MULTIKEY);
 

fftools/ffmpeg_demux.c

@@ -1259,6 +1259,11 @@ static int ist_add(const OptionsContext *o, Demuxer *d, AVStream *st)
     if (o->bitexact)
         av_dict_set(&ist->decoder_opts, "flags", "+bitexact", AV_DICT_MULTIKEY);
 
+    /* Attached pics are sparse, therefore we would not want to delay their decoding
+     * till EOF. */
+    if (ist->st->disposition & AV_DISPOSITION_ATTACHED_PIC)
+        av_dict_set(&ist->decoder_opts, "thread_type", "-frame", 0);
+
     switch (par->codec_type) {
     case AVMEDIA_TYPE_VIDEO:
         MATCH_PER_STREAM_OPT(frame_rates, str, framerate, ic, st);