https://blog.wongcw.com/2019/08/07/ffmpeg-4-2-%e7%99%bc%e5%b8%83%ef%bc%9aav1-%e8%a7%a3%e7%a2%bc%e6%94%af%e6%8c%81gif-%e8%a7%a3%e6%9e%90%e5%99%a8/