Margin gets applied after Width/Height

[grokys]

The following test fails:

        [Fact]
        public void Margin_Should_Be_Applied_Before_Width_Height()
        {
            MeasureTest target;

            var outer = new Decorator
            {
                Width = 100,
                Height = 100,
                Child = target = new MeasureTest
                {
                    Width = 80,
                    Height = 80,
                    Margin = new Thickness(10),
                }
            };

            outer.Measure(Size.Infinity);

            Assert.Equal(new Size(80, 80), target.AvailableSize);
        }

The available size should be 80,80 but gets passed as 60,60. I suspect this is the problem @kekekeks is seeing in #984.