When we're unfullscreening, we might be returning to a window state that
has its size either managed by constraints (tiled, maximized), or not
(floating). Lets just pass the configure size 0x0 when we're not using
constrained sizes (i.e. the window going from being fullscreen to not
maximized) and let the application decide how to size itself.
Fixes: https://gitlab.gnome.org/GNOME/mutter/issues/638https://gitlab.gnome.org/GNOME/mutter/merge_requests/621