gtk/window: Check widget mappdness and not surface mappedness

gtk/window: Check widget mappdness and not surface mappedness

When deciding whether to call gdk_toplevel_present(), check the
mappedness of GtkWidget instead of the mapped-ness of GdkSurface, as the
latter is mapped asynchronously, while the former is direct state of the
GtkWindow itself.

This fixes an issue where calling e.g. gtk_window_maximize() shortly
after showing it didn't take effect, as hadn't been mapped yet. While
maximizing after showing is racy, and will likely glitch from time to
time, the correct thing to do is still to maximize and handle whatever
state the compositor configured the window to be in.

Closes: https://gitlab.gnome.org/GNOME/gtk/-/issues/3466