[8] ffmpeg + SDL2 实现的视频播放器「快进、快退」

目标： 1. 实现快进、快退。 ###一些知识： * ffmpeg中定义： ``` #define AV_TIME_BASE 1000000 #define AV_TIME_BASE_Q (AVRational){1, AV_TIME_BASE} #define AVSEEK_FLAG_BACKWARD 1 //向后查找标志。 #define AVSEEK_FLAG_BYTE 2 #define AVSEEK_FLAG_ANY 4 #define AVSEEK_FLAG_FRAME 8 ``` *函数av_rescale_q ``` int64_t av_rescale_q(int64_t a, AVRational bq, AVRational cq) Returns a * bq / cq. ``` * 函数av_seek_frame ``` int av_seek_frame(AVFormatContext *s, int stream_index, int64_t timestamp, int flags) Seeks to the key frame at timestamp. stream_index: If stream_index is -1, a default stream is selected, and timestamp is automatically converted from AV_TIME_BASE units to the stream specific time_base. timestamp is measured in AVStream.time_base units or if there is no stream specified then in AV_TIME_BASE units. flags: Set options regarding direction and seeking mode. AVSEEK_FLAG_ANY: Seek to any frame, not just keyframes AVSEEK_FLAG_BACKWARD: Seek backward AVSEEK_FLAG_BYTE: Seeking based on position in bytes ```