void_stdcall paint_video_wnd(void* client, HWND hwnd);
client:[in]，The client object which is form new_client
hwnd:[in], The window's handle which is showing the video
void
If user want to show the last frame on the window who call add_video_wnd and set the "auto_paint" FALSE, can call this interface.(usually, it's called in OnPaint function)