Update CImg to 3.3.6 (#4275)

packages/c/cimg/xmake.lua

@@ -8,6 +8,8 @@ package("cimg")
         return version:gsub("%v", "v.")
     end})
     add_urls("https://github.com/greyclab/cimg.git")
+
+    add_versions("v3.3.6", "7bb6621c38458152f3d1cae3f020e4ca6a314076cb7b4b5d6bbf324ad3d0ab88")
     add_versions("v3.2.6", "1fcca9a7a453aa278660c10d54c6db9b4c614b6a29250adeb231e95a0be209e7")
 
     if is_plat("windows") then