Address review comments

Author: Evangelink

src/ImageSharp/Processing/Processors/Transforms/ISwizzler.cs

@@ -17,7 +17,7 @@ public interface ISwizzler
         /// Applies the swizzle transformation to a given point.
         /// </summary>
         /// <param name="point">Point to transform.</param>
-        /// <param name="newPoint">The transformed point.</param>
-        void Transform(Point point, out Point newPoint);
+        /// <returns>The transformed point.</returns>
+        Point Transform(Point point);
     }
 }

src/ImageSharp/Processing/Processors/Transforms/SwizzleProcessor{TSwizzler,TPixel}.cs

@@ -29,10 +29,11 @@ protected override void OnFrameApply(ImageFrame<TPixel> source, ImageFrame<TPixe
             Point newPoint;
             for (p.Y = 0; p.Y < source.Height; p.Y++)
             {
+                Span<TPixel> rowSpan = source.GetPixelRowSpan(p.Y);
                 for (p.X = 0; p.X < source.Width; p.X++)
                 {
-                    this.swizzler.Transform(p, out newPoint);
-                    destination[newPoint.X, newPoint.Y] = source[p.X, p.Y];
+                    newPoint = this.swizzler.Transform(p);
+                    destination[newPoint.X, newPoint.Y] = rowSpan[p.X];
                 }
             }
         }