Deprecate wxBitmap assignment operator from wxCursor in wxMSW.

This assignment operator is not portable and a better alternative is available
now with the addition of the wxBitmap ctor from wxCursor in the previous
commit.

See #15699.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77538 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/bitmap.h

@@ -110,12 +110,17 @@ public:
         return *this;
     }
 
+#ifdef WXWIN_COMPATIBILITY_3_0
+    // This assignment operator is not portable as it is not implemented in any
+    // other ports.
+    wxDEPRECATED_MSG("Don't assign wxCursor to an existing wxBitmap, create a new wxBitmap from wxCursor instead.")
     wxBitmap& operator=(const wxCursor& cursor)
     {
         (void)CopyFromCursor(cursor);
 
         return *this;
     }
+#endif // WXWIN_COMPATIBILITY_3_0
 
     virtual ~wxBitmap();