int _stdcall decode_video(void * ctx, VIDEO_FRAME * vf, unsigned char ** rgb);
ctx:[in]，The point to the context for decoding video frame which get from init_video_decoder
vf:[in]，The source data which is to decode
rgb:[in]，The dest RGB data which is from decoding
>0, success, is the RGB data's lenght, esle is failed
Decode the video frame