Fix and test margin layout.

Perspex.Direct2D1.RenderTests/Controls/BorderTests.cs

@@ -120,5 +120,29 @@ namespace Perspex.Direct2D1.RenderTests.Controls
             this.RenderToFile(target);
             this.CompareImages();
         }
+
+        [TestMethod]
+        public void Border_Margin_Offsets_Content()
+        {
+            Decorator target = new Decorator
+            {
+                Padding = new Thickness(8),
+                Width = 200,
+                Height = 200,
+                Content = new Border
+                {
+                    BorderBrush = Brushes.Black,
+                    BorderThickness = 2,
+                    Content = new Border
+                    {
+                        Background = Brushes.Red,
+                        Margin = new Thickness(2),
+                    }
+                }
+            };
+
+            this.RenderToFile(target);
+            this.CompareImages();
+        }
     }
 }

Perspex/Controls/Control.cs

@@ -327,9 +327,11 @@ namespace Perspex.Controls
 
         public void Arrange(Rect rect)
         {
+            Thickness margin = this.Margin;
+
             this.Bounds = new Rect(
-                rect.Position,
-                this.ArrangeContent(rect.Size.Deflate(this.Margin).Constrain(rect.Size)));
+                new Point(rect.Position.X + margin.Left, rect.Position.Y + margin.Top),
+                this.ArrangeContent(rect.Size.Deflate(margin).Constrain(rect.Size)));
         }
 
         public void Measure(Size availableSize)