fftools/ffmpeg: move post-demux packet processing to ffmpeg_demux

That is a more appropriate place for this code and will allow hiding
more of InputStream.

The value of repeat_pict extracted from libavformat internal parser no
longer needs to be trasmitted outside of the demuxing thread.

Move readrate handling to the demuxer thread. This has to be done in the
same commit, since it reads InputStream.dts,nb_packets, which are now
set in the demuxer thread.

fftools/ffmpeg.h

@@ -380,12 +380,6 @@ typedef struct InputStream {
 
     int           wrap_correction_done;
 
-    // the value of AVCodecParserContext.repeat_pict from the AVStream parser
-    // for the last packet returned from ifile_get_packet()
-    // -1 if unknown
-    // FIXME: this is a hack, the avstream parser should not be used
-    int last_pkt_repeat_pict;
-
     int64_t filter_in_rescale_delta_last;
 
     // when forcing constant input framerate through -r,