Fix setting transparency flag for wxBitmap created from wxIcon

wxBitmap should be made transparency-capable before copying wxIcon bits to it. Doing so when wxIcon bits are already copied seems to work fine for OSX 10.8, but not for newer versions. 

Closes #17953.

src/osx/core/bitmap.cpp

@@ -839,13 +839,14 @@ bool wxBitmap::CopyFromIcon(const wxIcon& icon)
 
     if ( Create( w, h ) )
     {
+        // Assume 32 bpp icon has transparency values
+        UseAlpha(icon.GetDepth() == 32);
+
         wxMemoryDC dc ;
         dc.SelectObject( *this ) ;
         dc.DrawIcon( icon , 0 , 0 ) ;
         dc.SelectObject( wxNullBitmap ) ;
 
-        // Assume 32 bpp icon has transparency values
-        UseAlpha(icon.GetDepth() == 32);
         return true;
     }