Fix adding wxBitmap with mask to generic wxImageList

If wxBitmap with mask is added to wxImageList that doesn't support masks
we need to convert a bitmap mask to alpha channel values prior to adding
bitmap to the list to preserve bitmap transparency.

include/wx/generic/imaglist.h

@@ -58,6 +58,7 @@ private:
     const wxBitmap *DoGetPtr(int index) const;
 
     wxVector<wxBitmap> m_images;
+    bool m_useMask;
 
     // Size of a single bitmap in the list.
     wxSize m_size;