Add some comments to wxGtkImage

Document which pointers can, and can't, be null and when exactly is the
stored bitmap valid, as this is far from being immediately obvious.

No real changes.

include/wx/gtk/private/image.h

@@ -23,9 +23,13 @@ public:
     static GtkWidget* New(wxWindow* win = NULL);
     void Set(const wxBitmap& bitmap);
 
+    // This pointer is never null and is owned by this class.
     BitmapProvider* m_provider;
 
     wxDECLARE_NO_COPY_CLASS(wxGtkImage);
+
+    // This class is constructed by New() and destroyed by its GObject
+    // finalizer, so neither its ctor nor dtor can ever be used.
     wxGtkImage() wxMEMBER_DELETE;
     ~wxGtkImage() wxMEMBER_DELETE;
 };