[gtk3] 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 window 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.
Related: #4634 (closed)