Set AVCodecContext->codec upon initialization.

As far as i understand, currently we have to initialize AVCodecContext->codec manually, in order to be able to call av_opt_set.

    AVStream* stream = avformat_new_stream(output_context, codec);
    stream->codec->codec = codec; // // Otherwise next line fails, because stream->codec->codec is null
    av_opt_set( stream->codec, "tune", "zerolatency", AV_OPT_SEARCH_CHILDREN ); 

That's weird because avcodec_get_context_defaults3 has AVCodec* in parameter.

FATE ok with that.