ffmpeg调用顺序
1.av_register_all
2.avformat_network_init
3.avformat_open_input
4.avformat_find_stream_info
5.av_find_best_stream
6.avcodec_find_decoder
7.avcodec_alloc_context3
8.avcodec_parameters_to_context
9.avcodec_open2
10.av_frame_alloc
11.av_read_frame
12.avcodec_send_packet
13.avcodec_receive_frame