Fixed CAOpenGLLayer example being rendered as solid red

The SimpleCocoaVTK example project has two kinds of NSViews, one being based on CAOpenGLLayer. That one has been broken in master for some time now, rendering as solid red. Fixed by invoking InitializeFromCurrentContext(). Bypassed vtkCocoaRenderWindow's implementation though because it mutates the NSView and NSWindow ivars, which is undesirable in this case. Instead, invoke super's implementation.

Examples/GUI/Cocoa/CustomLayer.mm

@@ -24,9 +24,10 @@
 
   // Tell VTK to render.
   assert([customView renderWindowInteractor] -> GetInitialized());
-  vtkRenderWindow* renderWindow = [customView renderWindow];
+  vtkCocoaRenderWindow* renderWindow = [customView renderWindow];
   if (renderWindow && renderWindow->GetMapped())
   {
+    renderWindow->Superclass::InitializeFromCurrentContext();
     renderWindow->Render();
   }