Give a warning when using default scale factor in wxGtkImage

This is not supposed to happen, but warn about it if it nevertheless
does.

src/gtk/image_gtk.cpp

@@ -7,6 +7,7 @@
 
 #include "wx/wxprec.h"
 
+#include "wx/log.h"
 #include "wx/window.h"
 
 #include "wx/gtk/private/wrapgtk.h"
@@ -46,6 +47,12 @@ double BitmapProviderDefault::GetScale() const
     {
         return m_win->GetDPIScaleFactor();
     }
+
+    // We expect to always have a window by the time this function is called,
+    // so while we try to do something reasonable even if we don't have it,
+    // at least log it because this is not expected to happen.
+    wxLogDebug("No window in wxGtkImage, using main window scale.");
+
 #if GTK_CHECK_VERSION(3,10,0)
     return gdk_window_get_scale_factor(wxGetTopLevelGDK());
 #else