implement window invalidation.

src/Avalonia.Native.OSX/window.mm

@@ -58,6 +58,11 @@ public:
         return S_OK;
     }
     
+    virtual void Invalidate (AvnRect rect)
+    {
+        [View setNeedsDisplayInRect:[View frame]];
+    }
+    
 protected:
     virtual NSWindowStyleMask GetStyle()
     {

src/Avalonia.Native/WindowImplBase.cs

@@ -139,7 +139,7 @@ namespace Avalonia.Native
 
         public void Invalidate(Rect rect)
         {
-            //TODO;
+            _native.Invalidate(new AvnRect { Height = rect.Height, Width = rect.Width, X = rect.X, Y = rect.Y });
         }
 
         public void SetInputRoot(IInputRoot inputRoot)

src/headers/avalonia-native.h

@@ -68,6 +68,7 @@ AVNCOM(IAvnWindowBase, 02) : virtual IUnknown
     virtual HRESULT Close() = 0;
     virtual HRESULT GetClientSize(AvnSize*ret) = 0;
     virtual HRESULT Resize(double width, double height) = 0;
+    virtual void Invalidate (AvnRect rect) = 0;
 };
 
 AVNCOM(IAvnWindow, 03) : virtual IAvnWindowBase