[FFmpeg-user] Proper wait for AVStream fully configurated

[Большой Человек]

I am developing a camera streaming service. And sometimes I have an issue
when I got wrong AVCodecParameters::format during initialization for
cameras with slow connection, or it happens for cameras with h264+ codec
with rare key packets.

How it happens.
This piece of code successes:
```

AVDictionary *opts = NULL;
if (config_->uset_tcp_transport)
    av_dict_set(&opts, "rtsp_transport", "tcp", 0);
av_dict_set(&opts, "stimeout", std::to_string(starting_ttl_sec_ *
1000000).c_str(), 0);

avformat_open_input(&fmtc_, rtsp_url, NULL, &opts);

video_stream_index_ = av_find_best_stream(fmtc_, AVMEDIA_TYPE_VIDEO,
-1, -1, &codec_, 0);
audio_stream_index_ = av_find_best_stream(fmtc_, AVMEDIA_TYPE_AUDIO,
-1, -1, &codec_, 0);

```
Then sometimes I get wrong value, when I check pixel format:

chroma_format_ = (AVPixelFormat)
fmtc_->streams[video_stream_index_]->codecpar->format;


But if I try to get a packet first

av_read_frame(fmtc_, pkt_);

Then "chroma_format_" most certainly will be correct.


I assume it is connected with getting first key packet. Why and how
can I be sure that codecpair is fully configurated?