fftools/ffmpeg: stop using AVStream.nb_frames in do_video_stats()

Its use for muxing is not documented, in practice it is incremented per
each packet successfully passed to the muxer's write_packet(). Since
there is a lot of indirection between ffmpeg receiving a packet from the
encoder and it actually being written (e.g. bitstream filters, the
interleaving queue), using nb_frames here is incorrect.

Add a new counter for packets received from encoder instead.

fftools/ffmpeg.c

@@ -851,7 +851,7 @@ static void do_video_stats(OutputStream *ost, int frame_size)
     }
 
     enc = ost->enc_ctx;
-    frame_number = ost->st->nb_frames;
+    frame_number = ost->packets_encoded;
     if (vstats_version <= 1) {
         fprintf(vstats_file, "frame= %5d q= %2.1f ", frame_number,
                 ost->quality / (float)FF_QP2LAMBDA);
@@ -950,6 +950,8 @@ static int encode_frame(OutputFile *of, OutputStream *ost, AVFrame *frame)
         if (enc->codec_type == AVMEDIA_TYPE_VIDEO && vstats_filename)
             do_video_stats(ost, pkt->size);
 
+        ost->packets_encoded++;
+
         output_packet(of, pkt, ost, 0);
     }
 

fftools/ffmpeg.h

@@ -551,6 +551,8 @@ typedef struct OutputStream {
     // number of frames/samples sent to the encoder
     uint64_t frames_encoded;
     uint64_t samples_encoded;
+    // number of packets received from the encoder
+    uint64_t packets_encoded;
 
     /* packet quality factor */
     int quality;