add back unsafe optimizations in ErrorDither

src/ImageSharp/Processing/Processors/Dithering/ErrorDither.cs

@@ -110,19 +110,13 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
 
             for (int y = bounds.Top; y < bounds.Bottom; y++)
             {
-                // Unsafe optimizations undone temporarily.
-                // Sporadic local AccessViolationException indicates possible indexing bug.
-                // ref TPixel sourceRowRef = ref MemoryMarshal.GetReference(source.DangerousGetRowSpan(y));
-                // ref byte destinationRowRef = ref MemoryMarshal.GetReference(destination.GetWritablePixelRowSpanUnsafe(y - offsetY));
-                Span<TPixel> sourceSpan = sourceBuffer.DangerousGetRowSpan(y);
-                Span<byte> destSpan = destination.GetWritablePixelRowSpanUnsafe(y - offsetY);
+                ref TPixel sourceRowRef = ref MemoryMarshal.GetReference(sourceBuffer.DangerousGetRowSpan(y));
+                ref byte destinationRowRef = ref MemoryMarshal.GetReference(destination.GetWritablePixelRowSpanUnsafe(y - offsetY));
 
                 for (int x = bounds.Left; x < bounds.Right; x++)
                 {
-                    // TPixel sourcePixel = Unsafe.Add(ref sourceRowRef, x);
-                    // Unsafe.Add(ref destinationRowRef, x - offsetX) = quantizer.GetQuantizedColor(sourcePixel, out TPixel transformed);
-                    TPixel sourcePixel = sourceSpan[x];
-                    destSpan[x - offsetX] = quantizer.GetQuantizedColor(sourcePixel, out TPixel transformed);
+                    TPixel sourcePixel = Unsafe.Add(ref sourceRowRef, x);
+                    Unsafe.Add(ref destinationRowRef, x - offsetX) = quantizer.GetQuantizedColor(sourcePixel, out TPixel transformed);
                     this.Dither(source, bounds, sourcePixel, transformed, x, y, scale);
                 }
             }