使用 opencv VideoWriter 保存视频流，只有6k

官网示例代码：

fourcc = cv.VideoWriter_fourcc(*'XVID')
out = cv.VideoWriter('output.avi', fourcc, 20.0, (640,  480))

修改为：

size = (int(cap.get(cv.CAP_PROP_FRAME_WIDTH)),
        int(cap.get(cv.CAP_PROP_FRAME_HEIGHT)))
out = cv.VideoWriter('output.avi', cv.VideoWriter_fourcc(*'XVID'), 20.0, size, 1)

即可保存视频