Faster pixel access [skip ci]

Former-commit-id: 6acf8d3a4de0287b22205537591bdf3517f4fc32
Former-commit-id: 11a91852b54bf2e42182cfe6bf08a0fdb969a84f
Former-commit-id: 05574a79bfe33a798d9dfff869e712a276e8ee64

src/ImageProcessorCore/Image/PixelAccessor.cs

@@ -104,8 +104,8 @@ namespace ImageProcessorCore
         /// <returns>The <see cref="TColor"/> at the specified position.</returns>
         public TColor this[int x, int y]
         {
-            get { return Unsafe.Read<TColor>(pixelsBase + ((y * this.RowStride) + (x * this.PixelSize))); }
-            set { Unsafe.Write(pixelsBase + ((y * this.RowStride) + (x * this.PixelSize)), value); }
+            get { return Unsafe.Read<TColor>(this.pixelsBase + (y * this.Width + x) * Unsafe.SizeOf<TColor>()); }
+            set { Unsafe.Write(this.pixelsBase + (y * this.Width + x) * Unsafe.SizeOf<TColor>(), value); }
         }
 
         /// <summary>