make it parallel again!

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

@@ -110,13 +110,21 @@ namespace ImageSharp.Processing.Processors
                 {
                     firstPassPixels.Clear();
 
-                    using (PinnedBuffer<Vector4> tempRowBuffer = new PinnedBuffer<Vector4>(sourcePixels.Width))
+                    Parallel.For(
+                        0,
+                        sourceRectangle.Bottom,
+                        this.ParallelOptions,
+                        y =>
                             {
-                        for (int y = 0; y < sourceRectangle.Bottom; y++)
+                                // TODO: Without Parallel.For() this buffer object could be reused:
+                                using (PinnedBuffer<Vector4> tempRowBuffer = new PinnedBuffer<Vector4>(sourcePixels.Width))
                                 {
                                     BufferSpan<TColor> sourceRow = sourcePixels.GetRowSpan(y);
 
-                            BulkPixelOperations<TColor>.Instance.ToVector4(sourceRow, tempRowBuffer, sourceRow.Length);
+                                    BulkPixelOperations<TColor>.Instance.ToVector4(
+                                        sourceRow,
+                                        tempRowBuffer,
+                                        sourceRow.Length);
 
                                     if (this.Compand)
                                     {
@@ -135,10 +143,14 @@ namespace ImageSharp.Processing.Processors
                                         }
                                     }
                                 }
-                    }
+                            });
 
                     // Now process the rows.
-                    for (int y = minY; y < maxY; y++)
+                    Parallel.For(
+                        minY,
+                        maxY,
+                        this.ParallelOptions,
+                        y =>
                         {
                             // Ensure offsets are normalised for cropping and padding.
                             WeightsWindow window = this.VerticalWeights.Weights[y - startY];
@@ -167,7 +179,7 @@ namespace ImageSharp.Processing.Processors
                                     targetPixels[x, y] = d;
                                 }
                             }
-                    }
+                        });
                 }
 
                 source.SwapPixelsBuffers(targetPixels);