int _stdcall play_video(void* client, unsigned int stream);
client:[in]，The client object which is form new_client
stream:[in]，The video stream number which will be played
Return ERROR_OK if success，else refer to the definition of error number
Play the stream of the camera, make sure the camera has connect now.
It will occur a VIDEO_STATUS_CHANGED event, usually,
the video status will follow the order-- STOPPED -> REQUESTING -> PLAYING.
When play video successfully, the camera will send video data to user by VIDEO_DATA event.