编码器设置，去除B帧的方法

1. 方法一

c->max_b_frames = 0;   
ret = avcodec_open2(c, codec, NULL);

2. 方法二

AVDictionary *d = NULL;
av_dict_set_int(&d,"bf",0,0);
ret = avcodec_open2(c, codec, &d);

3. 方法三

AVDictionary *d = NULL;
av_dict_set(&d,"x264opts","bframe=0",0);    //实际验证未成功，todo
ret = avcodec_open2(c, codec, &d);