ffmplay---seek实现播放进度控制

0、实现播放进度控制—即实现随机访问流媒体文件

seek流程：
1、停下demux线程、音视频解码线程
2、清空pkt队列
3、恢复三个线程（此时还不能进入正常播放状态）
4、扔掉小于seek的目标pts的帧
5、直到拿到帧的pts≥seek目标pts，才恢复音视频渲染
6、恢复正常播放

ffplay的seek操作：对于ts和mp4的逻辑差异
mp4 可以seek到指定的时间戳
ts 是 seek到文件的某个position，而不能直接seek到指定的时间点

1、seek操作要点

1、操作基准：即seek操作当前时间点，因此在进行播放时需要实时记录音视频流的DTS或者PTS
2、时间单位：进行seek操作时，根据流索引，确定其时间单位time_base，
3、时间大小：即向前或者向后seek的时间大小，根据此时间大小和当前时间点，可以计算出向前或者向后进行seek操作时的目标时间
4、操作载体：即是按照音频还是视频进行seek操作，只需要按照其中一个流进行seek即可，不需要分别对音视频流进行seek操作
5、操作刷新：在进行seek操作之后，正常播放之前需要将编解码的内部缓冲区进行刷新，同时也要将自行控制的缓冲区内缓存的音视频清零

2、seek相关函数

avformat_seek_file（会调用av_seek_frame）

/**
 * Seek to timestamp ts.
 * Seeking will be done so that the point from which all active streams
 * can be presented successfully will be closest to ts and within min/max_ts.
 * Active streams are all streams that have AVStream.discard < AVDISCARD_ALL.
 *
 * If flags contain AVSEEK_FLAG_BYTE, then all timestamps are in bytes and
 * are the file position (this may not be supported by all demuxers).
 * If flags contain AVSEEK_FLAG_FRAME, then all timestamps are in frames
 * in the stream with stream_index (this may not be supported by all demuxers).
 * Otherwise all timestamps are in units of the stream selected by stream_index
 * or if stream_index is -1, in AV_TIME_BASE units.
 * If flags contain AVSEEK_FLAG_ANY, then non-keyframes are treated as
 * keyframes (this may not be supported by all demuxers).
 * If flags contain AVSEEK_FLAG_BACKWARD, it is ignored.
 *
 * @param s media file handle
 * @param stream_index index of the stream which is used as time base reference
 * @param min_ts smallest acceptable timestamp
 * @param ts target timestamp
 * @param max_ts largest acceptable timestamp
 * @param flags flags
 * @return >=0 on success, error code otherwise
 *
 * @note This is part of the new seek API which is still under construction.
 *       Thus do not use this yet. It may change at any time, do not