Call layer.setNeedsDisplay on show. (#17096)

The `AvaloniaNative.GlPlatformSurface.CreateGlRenderTarget` method can only be called on the UI thread. In normal circumstances this method is called in response to a call to `TopLevel.HandlePaint` from the native `AvnView.updateLayer` method. However, a customer was experiencing a problem where `AvnView.updateLayer` and by extension `TopLevel.HandlePaint` were being called before the window is shown. This broke the creation of the GL render target.

native/Avalonia.Native/src/OSX/WindowBaseImpl.mm

@@ -132,6 +132,13 @@ HRESULT WindowBaseImpl::Show(bool activate, bool isDialog) {
 
         _shown = true;
         [Window setCollectionBehavior:collectionBehavior];
+        
+        // Ensure that we call needsDisplay = YES so that AvnView.updateLayer is called after the
+        // window is shown: if the client is pumping messages during the window creation/show
+        // process, it's possible that updateLayer gets called after the window is created but
+        // before it's is shown.
+        [View.layer setNeedsDisplay];
+        
         return S_OK;
     }
 }