Reduce errors in AC strategy code

src/ImageSharp/Formats/Jxl/Processing/JxlAcStrategy.cs

@@ -81,9 +81,9 @@ internal struct JxlAcStrategy
 
     public readonly JxlAcStrategyType Strategy { get; }
 
-    public void ComputeNaturalCoefficientOrder(ref int order) => CoefficientOrderAndLookup(this, false, ref order);
+    public readonly void ComputeNaturalCoefficientOrder(Span<int> order) => CoefficientOrderAndLookup(this, false, order);
 
-    public void ComputeNaturalCoefficientOrderLookup(ref int lookup) => CoefficientOrderAndLookup(this, true, ref lookup);
+    public readonly void ComputeNaturalCoefficientOrderLookup(Span<int> lookup) => CoefficientOrderAndLookup(this, true, lookup);
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static int GetTypeBit(JxlAcStrategyType type) => 1 << (int)type;
@@ -97,7 +97,7 @@ internal struct JxlAcStrategy
         int cx = strategy.CoveredBlocksX;
         int cy = strategy.CoveredBlocksY;
 
-        CoefficientLayout(ref cx, ref cy);
+        JxlForwardCoefficientOrder.CoefficientLayout(ref cx, ref cy);
 
         int xs = cx / cy;
         int xsm = xs - 1;

src/ImageSharp/Formats/Jxl/Processing/JxlAcStrategyImage.cs

@@ -103,11 +103,11 @@ internal sealed class JxlAcStrategyImage : IDisposable
         return this.SetNoBoundsChecks(x, y, type, check: false);
     }
 
-    public void FillDct8(in Rectangle rect) => this.FillPlane(((int)JxlAcStrategyType.DCT << 1) | 1, this.layers, in rect);
+    public void FillDct8(in Rectangle rect) => JxlImageOperations.FillPlane((byte)(((int)JxlAcStrategyType.DCT << 1) | 1), this.layers ?? throw new InvalidOperationException("Image is missing"), rect);
 
     public void FillDct8() => this.FillDct8(this.layers!.GetRectangle());
 
-    public void FillInvalid() => this.FillImage(Invalid, this.layers);
+    public void FillInvalid() => JxlImageOperations.FillImage(Invalid, this.layers ?? throw new InvalidOperationException("Image is missing"));
 
     public void Dispose()
     {