Optimize for when not using nested cache folders

Author: ronaldbarendse

src/ImageSharp.Web/Caching/PhysicalFileSystemCache.cs

@@ -152,6 +152,12 @@ private string ToImageFilePath(string path, in ImageCacheMetadata metaData)
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         internal static unsafe string ToFilePath(string key, int cacheFolderDepth)
         {
+            if (cacheFolderDepth == 0)
+            {
+                // Short-circuit when not nesting folders
+                return key;
+            }
+
             int length;
             int nameStartIndex;
             if (cacheFolderDepth >= key.Length)