[Xwt.Wpf] Fix commit introduced with pull request #440.

The change didn't account for case where widget can be embedded into a
native WPF context and thus the Frontend ParentWindow property being never
set causing a infinite loop of drawing refresh.

Xwt.WPF/Xwt.WPFBackend/CanvasBackend.cs

@@ -14,6 +14,7 @@ namespace Xwt.WPFBackend
 		: WidgetBackend, ICanvasBackend
 	{
 		#region ICanvasBackend Members
+		bool hasInitialParent;
 
 		public CanvasBackend ()
 		{
@@ -41,10 +42,11 @@ namespace Xwt.WPFBackend
 		{
 			// delay drawing until all parents are registered and we can
 			// get a window which is required for many drawing operations
-			if (Frontend.ParentWindow == null) {
+			if (!hasInitialParent && Widget.GetParentWindow () == null) {
 				QueueDraw ();
 				return;
 			}
+			hasInitialParent = true;
 
 			if (BackgroundColorSet) {
 				SolidColorBrush mySolidColorBrush = new SolidColorBrush ();

Xwt.WPF/Xwt.WPFBackend/Util.cs

@@ -64,5 +64,18 @@ namespace Xwt.WPFBackend
 
 			throw new InvalidOperationException("Invalid alignment value: " + alignment);
         }
+
+		public static System.Windows.Window GetParentWindow (this FrameworkElement element)
+		{
+			FrameworkElement current = element;
+			while (current != null) {
+				if (current is System.Windows.Window)
+					return (System.Windows.Window)current;
+
+				current = VisualTreeHelper.GetParent (current) as FrameworkElement;
+			}
+
+			return null;
+		}
     }
 }

Xwt.WPF/Xwt.WPFBackend/WidgetBackend.cs

@@ -674,15 +674,7 @@ namespace Xwt.WPFBackend
 
 		private SW.Window GetParentWindow()
 		{
-			FrameworkElement current = Widget;
-			while (current != null) {
-				if (current is SW.Window)
-					return (SW.Window)current;
-
-				current = VisualTreeHelper.GetParent (current) as FrameworkElement;
-			}
-
-			return null;
+			return Widget.GetParentWindow ();
 		}
 
 		public void DragStart (DragStartData data)