wayland: Check workspace position when preferred deco mode changes

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.