minor optimization

src/ImageSharp/Processing/Processors/Transforms/Resize/ResizeProcessor.cs

@@ -269,9 +269,11 @@ namespace SixLabors.ImageSharp.Processing.Processors.Transforms
 
                     ref Vector4 tempRowBase = ref MemoryMarshal.GetReference(tempColSpan);
 
+                    int top = kernel.StartIndex - resizeWindow.Top;
+
                     for (int x = 0; x < width; x++)
                     {
-                        Span<Vector4> firstPassColumn = resizeWindow.GetColumnSpan(x, kernel.StartIndex);
+                        Span<Vector4> firstPassColumn = resizeWindow.GetColumnSpan(x).Slice(top);
 
                         // Destination color components
                         Unsafe.Add(ref tempRowBase, x) = kernel.ConvolveCore(firstPassColumn);

src/ImageSharp/Processing/Processors/Transforms/Resize/ResizeWindow.cs

@@ -85,6 +85,12 @@ namespace SixLabors.ImageSharp.Processing.Processors.Transforms
             return this.buffer.GetRowSpan(x).Slice(startY - this.Top);
         }
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public Span<Vector4> GetColumnSpan(int x)
+        {
+            return this.buffer.GetRowSpan(x);
+        }
+
         public void Initialize()
         {
             this.Initialize(0, this.windowHeight);