从摄像头中读取视频，可退出，并保存为图片-Read the video the camera, can exit, and saved as a picture

这段代码主要适合流媒体方面的初学者，包含两个工程：录制屏幕以及抓取摄像头画面(This code is mainly suitable for streaming media beginners, including two projects: recording screen and grab the camera screen)