今天突然遇到了这么个奇怪的问题，之前一直没有注意到。
imshow()之后要加waitKey()才能正确显示图像。
官方说明如下：
A common mistake for opencv newcomers is to call cv::imshow() in a loop through video frames, without following up each draw with cv::waitKey(30). In this case, nothing appears on screen, because highgui is never given time to process the draw requests from
cv::imshow().