Fix adding wxBitmap with mask to wxImageList not supporting masks (wxMSW)

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 native list to preserve bitmap transparency.

Closes #19036.

include/wx/msw/imaglist.h

@@ -194,6 +194,7 @@ public:
 protected:
   WXHIMAGELIST m_hImageList;
   wxSize m_size;
+  bool m_useMask;
 
   wxDECLARE_DYNAMIC_CLASS_NO_COPY(wxImageList);
 };