Apply margin before width/height.

Fixes #993.

src/Avalonia.Layout/Layoutable.cs

@@ -456,10 +456,9 @@ namespace Avalonia.Layout
 
                 ApplyTemplate();
 
-                var constrained = LayoutHelper
+                var constrained = LayoutHelper.ApplyLayoutConstraints(
-                    .ApplyLayoutConstraints(this, availableSize)
+                    this,
-                    .Deflate(margin);
+                    availableSize.Deflate(margin));
-
                 var measured = MeasureOverride(constrained);
 
                 var width = measured.Width;