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如何升级SDL：
去SDL2[官网](http://www.libsdl.org/download-2.0.php)下载最新SDL库，并将include和lib文件夹覆盖过来即可。替换时需要注意，.gitignore文件中忽略了.dll和.lib文件，提交.dll和.lib文件需要强制提交一下。
如图：
