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.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;