目前多数的直播播放是使用openGLES来实现的，使用常用的
UIWindow *mainWindow = [UIApplication sharedApplication].keyWindow;
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextFillRect(context, bounds);
CGContextTranslateCTM(context, shrinkSize.width, shrinkSize.height);
[mainWindow.layer renderInContext:context];
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
不能截取播放器播放的画面，只能截取到一块黑屏
目前使用下面的方法可以直接截取
UIWindow *mainWindow = [UIApplication sharedApplication].keyWindow;
UIGraphicsBeginImageContextWithOptions(mainWindow.frame.size, NO, 0);
[mainWindow drawViewHierarchyInRect:mainWindow.frame afterScreenUpdates:YES];
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
