Fix overflow artifacts

src/ImageSharp/Processing/Processors/Normalization/AutoLevelProcessor{TPixel}.cs

@@ -182,14 +182,14 @@ internal class AutoLevelProcessor<TPixel> : HistogramEqualizationProcessor<TPixe
             ref int cdfBase = ref MemoryMarshal.GetReference(this.cdfBuffer.GetSpan());
             var sourceAccess = new PixelAccessor<TPixel>(this.source);
             Span<TPixel> pixelRow = sourceAccess.GetRowSpan(y);
-            int levels = this.luminanceLevels;
+            int levelsMinusOne = this.luminanceLevels - 1;
             float noOfPixelsMinusCdfMin = this.numberOfPixelsMinusCdfMin;
 
             for (int x = 0; x < this.bounds.Width; x++)
             {
                 // TODO: We should bulk convert here.
                 ref TPixel pixel = ref pixelRow[x];
-                var vector = pixel.ToVector4() * levels;
+                var vector = pixel.ToVector4() * levelsMinusOne;
 
                 uint originalX = (uint)MathF.Round(vector.X);
                 float scaledX = Unsafe.Add(ref cdfBase, originalX) / noOfPixelsMinusCdfMin;

tests/Images/External/ReferenceOutput/HistogramEqualizationTests/AutoLevel_SeparateChannels_CompareToReferenceOutput_Rgba32_forest_bridge.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:25041d2dafe6c01cfec0ae3c2ec15046accd44c02b737a4cfa464ad5f61d01af
+oid sha256:aada4a2ccf45de24f2a591a18d9bc0260ceb3829e104fee6982061013ed87282
 size 14107709