Added LinearGradient vertical fill tests.

tests/Perspex.RenderTests/Media/LinearGradientBrushTests.cs

@@ -23,7 +23,7 @@ namespace Perspex.Direct2D1.RenderTests.Media
         }
 
         [Fact]
-        public void LinearGradientBrush_RedBlue_Fill()
+        public void LinearGradientBrush_RedBlue_Horizontal_Fill()
         {
             Decorator target = new Decorator
             {
@@ -48,5 +48,32 @@ namespace Perspex.Direct2D1.RenderTests.Media
             RenderToFile(target);
             CompareImages();
         }
+
+        [Fact]
+        public void LinearGradientBrush_RedBlue_Vertical_Fill()
+        {
+            Decorator target = new Decorator
+            {
+                Padding = new Thickness(8),
+                Width = 200,
+                Height = 200,
+                Child = new Border
+                {
+                    Background = new LinearGradientBrush
+                    {
+                        StartPoint = new Point(0.5, 0),
+                        EndPoint = new Point(0.5, 1),
+                        GradientStops =
+                        {
+                            new GradientStop { Color = Colors.Red, Offset = 0 },
+                            new GradientStop { Color = Colors.Blue, Offset = 1 }
+                        }
+                    }
+                }
+            };
+
+            RenderToFile(target);
+            CompareImages();
+        }
     }
 }