fftools/ffmpeg_dec: eliminate all remaining InputStream uses

Previously, the demuxer would register decoder with the scheduler, using
InputStream as opaque, and pass the scheduling index to the decoder.

Now the registration is done by the decoder itself, using DecoderPriv as
opaque, and the scheduling index is returned to demuxer from dec_open().

decoder_thread() then no longer needs to be accessed from outside of
ffmpeg_dec and can be made static.

fftools/ffmpeg.h

@@ -290,6 +290,7 @@ enum DecoderFlags {
 #if FFMPEG_OPT_TOP
     DECODER_FLAG_TOP_FIELD_FIRST  = (1 << 3),
 #endif
+    DECODER_FLAG_SEND_END_TS      = (1 << 4),
 };
 
 typedef struct DecoderOpts {
@@ -756,8 +757,11 @@ AVBufferRef *hw_device_for_filter(void);
 /**
  * @param dec_opts Dictionary filled with decoder options. Its ownership
  *                 is transferred to the decoder.
+ *
+ * @retval ">=0" non-negative scheduler index on success
+ * @retval "<0"  an error code on failure
  */
-int dec_open(Decoder **pdec, Scheduler *sch, unsigned sch_idx,
+int dec_open(Decoder **pdec, Scheduler *sch,
              AVDictionary **dec_opts, const DecoderOpts *o);
 void dec_free(Decoder **pdec);
 
@@ -847,7 +851,6 @@ const char *opt_match_per_type_str(const SpecifierOptList *sol,
                                    char mediatype);
 
 void *muxer_thread(void *arg);
-void *decoder_thread(void *arg);
 void *encoder_thread(void *arg);
 
 #endif /* FFTOOLS_FFMPEG_H */