什么是体积云？
原来的云都是平面的，在天空盒上方放一张移动的图片。。。
而体积云则是有体积的，有高低的层次感。。这么理解不知道对不对
参考1：https://www.bilibili.com/read/cv18575367
参考2：https://zhuanlan.zhihu.com/p/485899538
参考3：https://www.bilibili.com/video/BV1iA411x7EN/
https://www.bilibili.com/video/BV1NS4y1n7Xd/
学习与参考资源：
http://killzone.dl.playstation.net/killzone/horizonzerodawn/presentations/Siggraph15_Schneider_Real-Time_Volumetric_Cloudscapes_of_Horizon_Zero_Dawn.pdf
http://www.diva-portal.org/smash/get/diva2:1223894/FULLTEXT01.pdf
https://www.programmer-books.com/wp-content/uploads/2019/07/GPU-Pro-7.pdf
http://advances.realtimerendering.com/s2017/Nubis%20-%20Authoring%20Realtime%20Volumetric%20Cloudscapes%20with%20the%20Decima%20Engine%20-%20Final%20.pdf
https://media.contentapi.ea.com/content/dam/eacom/frostbite/files/s2016_pbs_frostbite_sky_clouds.pdf
https://www.gamedev.net/forums/topic/680832-horizonzero-dawn-cloud-system/?page=6
https://dench.flatlib.jp/opengl/glsl_hlsl