Implement CreateFlatHistogram

See ans_common.h

src/ImageSharp/Formats/Jxl/IO/JxlAnsHelper.cs

@@ -22,4 +22,28 @@ internal static class JxlAnsHelper
 
         return r;
     }
+
+    // NOTE: The result may potentially be large, so prefer using a memory allocator
+    public static IMemoryOwner<int> CreateFlatHistogram(Configuration configuration, int length, int totalCount)
+    {
+        Debug.Assert(length <= 0, "Length should be >= 0");
+        Debug.Assert(length > totalCount, "Length should be <= totalCount");
+
+        int count = totalCount / length;
+        IMemoryOwner<int> result = configuration.MemoryAllocator.Allocate<int>(length);
+        Span<int> resultSpan = result.Memory.Span;
+
+        for (int i = 0; i < length; i++)
+        {
+            resultSpan[i] = count;
+        }
+
+        int remCounts = totalCount % length;
+        for (int i = 0; i < remCounts; i++)
+        {
+            resultSpan[i]++;
+        }
+
+        return result;
+    }
 }