Fix GeometryNode hit test.

And add to-do for RectangleNode geometry hit-test.

src/Avalonia.Visuals/Rendering/SceneGraph/GeometryNode.cs

@@ -39,7 +39,8 @@ namespace Avalonia.Rendering.SceneGraph
         public bool HitTest(Point p)
         {
             p *= Transform.Invert();
-            return Geometry.FillContains(p) || Geometry.StrokeContains(Pen, p);
+            return (Brush != null && Geometry.FillContains(p)) || 
+                (Pen != null && Geometry.StrokeContains(Pen, p));
         }
     }
 }

src/Avalonia.Visuals/Rendering/SceneGraph/RectangleNode.cs

@@ -49,6 +49,7 @@ namespace Avalonia.Rendering.SceneGraph
 
         public bool HitTest(Point p)
         {
+            // TODO: Only test interior when Brush != null.
             return (Rect * Transform).Contains(p);
         }
     }