avcodec_alloc_context3，avcodec_open2，avcodec_free_context，avcodec_close

avcodec_alloc_context3 是创建编解码器上下文，需要使用 avcodec_free_context释放

需要使用avcodec_free_context 释放

/**
 * Allocate an AVCodecContext and set its fields to default values. The
 * resulting struct should be freed with avcodec_free_context().
 *
 * @param codec if non-NULL, allocate private data and initialize defaults
 *              for the given codec. It is illegal to then call avcodec_open2()
 *              with a different codec.
 *              If NULL, then the codec-specific defaults won't be initialized,
 *              which may result in suboptimal default settings (this is
 *              important mainly for encoders, e.g. libx264).
 *
 * @return An AVCodecContext filled with def