gdk/wayland: fix gtk_window_set_default_size() after resize

This fixes Prompt/gnome-terminal to be able to properly resize a window programmatically after a user has dragged to resize the window.

This applies !is_resizing to size_is_fixed instead of fixed_size to avoid changing the branches leading to the code.

This definitely needs @jadahl to look over it to see if it even makes sense.