wayland: Don't always restore the saved size when floating

wayland: Don't always restore the saved size when floating

We only save the size when we transition from floating to fixed, so that
we can restore the size to the one prior to being fixed.

However, we should not restore to this size whenever we see a 0x0 size
from xdg_toplevel, as it can do that any time it doesn't care about the
size, e.g. when the surface is floating and just changing state.

Fix this by only using the saved size when transitioning from fixed to
floating, not when staying floating while previously floating.

Closes: https://gitlab.gnome.org/GNOME/gtk/-/issues/4634