add filter

[fangandyuan]

add filter, but inoperative.

- (kxMovieError) openVideoStream: (NSInteger) videoStream
{    
    // get a pointer to the codec context for the video stream
    AVCodecContext *codecCtx = _formatCtx->streams[videoStream]->codec;
    // find the decoder for the video stream
    AVCodec *codec = avcodec_find_decoder(codecCtx->codec_id);
    [_Tool setupFilters:@"crop=1/3*in_w:1/3*in_h" videoCodecCtx:codecCtx];