Fixed bug that created new instance of ScreenImpl every time it was accessed through WindowBaseImpl

src/Gtk/Avalonia.Gtk3/WindowBaseImpl.cs

@@ -342,7 +342,10 @@ namespace Avalonia.Gtk3
             Native.GtkWindowResize(GtkWidget, (int)value.Width, (int)value.Height);
         }
         
-        public IScreenImpl Screen { get; } = new ScreenImpl();
+        public IScreenImpl Screen
+        {
+            get;
+        } = new ScreenImpl();
 
         public Point Position
         {

src/OSX/Avalonia.MonoMac/WindowBaseImpl.cs

@@ -153,7 +153,10 @@ namespace Avalonia.MonoMac
             Position = pos;
         }
 
-        public IScreenImpl Screen { get; } = new ScreenImpl();
+        public IScreenImpl Screen
+        {
+            get;
+        } = new ScreenImpl();
 
         public override Point PointToClient(Point point)
         {

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -103,7 +103,11 @@ namespace Avalonia.Win32
             }
         }
 
-        public IScreenImpl Screen => new ScreenImpl();
+        public IScreenImpl Screen
+        {
+            get;
+        } = new ScreenImpl();
+
 
         public IRenderer CreateRenderer(IRenderRoot root)
         {