ffmpeg: ensure a keyframe was not seen before skipping packets

A keyframe could be buffered in the bsf and not be output until more packets
had been fed to it.

Signed-off-by: James Almer <jamrial@gmail.com>

fftools/ffmpeg.c

@@ -889,6 +889,8 @@ static void output_packet(OutputFile *of, AVPacket *pkt,
 
     /* apply the output bitstream filters */
     if (ost->bsf_ctx) {
+        if (pkt->flags & AV_PKT_FLAG_KEY)
+            ost->seen_kf = 1;
         ret = av_bsf_send_packet(ost->bsf_ctx, eof ? NULL : pkt);
         if (ret < 0)
             goto finish;
@@ -2026,7 +2028,7 @@ static void do_streamcopy(InputStream *ist, OutputStream *ost, const AVPacket *p
     }
 
     if ((!ost->frame_number && !(pkt->flags & AV_PKT_FLAG_KEY)) &&
-        !ost->copy_initial_nonkeyframes)
+        !ost->copy_initial_nonkeyframes && !ost->seen_kf)
         return;
 
     if (!ost->frame_number && !ost->copy_prior_start) {

fftools/ffmpeg.h

@@ -535,6 +535,7 @@ typedef struct OutputStream {
     int inputs_done;
 
     const char *attachment_filename;
+    int seen_kf;
     int copy_initial_nonkeyframes;
     int copy_prior_start;
     char *disposition;