This ensures that the window will have correct geometry if a maximized
window changes preferred decoration mode. X11Client does something
similar, see X11Client::updateShape().

In hindsight, perhaps, AbstractClient::{create,destroy}Decoration() must
preserve the old frame geometry, but it's not clear how to do that
because it requires decoration updates to be truly async, otherwise
there will be ugly flickering.


(cherry picked from commit 2560288e)