Use Interlocked.Increment during histogram calculation, fixes issue #1416

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

@@ -6,6 +6,7 @@ using System.Buffers;
 using System.Numerics;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
+using System.Threading;
 using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.PixelFormats;
@@ -51,7 +52,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
 
             using IMemoryOwner<int> histogramBuffer = memoryAllocator.Allocate<int>(this.LuminanceLevels, AllocationOptions.Clean);
 
-            // Build the histogram of the grayscale levels
+            // Build the histogram of the grayscale levels.
             var grayscaleOperation = new GrayscaleLevelsRowOperation(interest, histogramBuffer, source, this.LuminanceLevels);
             ParallelRowIterator.IterateRows(
                 this.Configuration,
@@ -114,7 +115,6 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
 #endif
             public void Invoke(int y)
             {
-                ref int histogramBase = ref MemoryMarshal.GetReference(this.histogramBuffer.GetSpan());
                 ref TPixel pixelBase = ref MemoryMarshal.GetReference(this.source.GetPixelRowSpan(y));
                 int levels = this.luminanceLevels;
 
@@ -123,7 +123,8 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
                     // TODO: We should bulk convert here.
                     var vector = Unsafe.Add(ref pixelBase, x).ToVector4();
                     int luminance = ImageMaths.GetBT709Luminance(ref vector, levels);
-                    Unsafe.Add(ref histogramBase, luminance)++;
+                    ref int histogramAtLuminance = ref MemoryMarshal.GetReference(this.histogramBuffer.Slice(luminance));
+                    Interlocked.Increment(ref histogramAtLuminance);
                 }
             }
         }