adjust so that mono bitmaps don't get an alpha channel

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@41964 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/mac/carbon/dcmemory.cpp

@@ -78,6 +78,7 @@ void wxMemoryDC::SelectObject( const wxBitmap& bitmap )
     if (m_selected.Ok())
     {
 #if wxMAC_USE_CORE_GRAPHICS
+        if ( m_selected.GetDepth() != 1 )
             m_selected.UseAlpha() ;
         void * data = m_selected.BeginRawAccess() ;