scene: Fix SurfaceItemWayland::freeze()

Vlad Zahorodniirequested to merge
work/zzag/scene-surface-item-freeze into master

When a wl_surface is unmapped, we need to stop updating the buffer in SurfacePixmapWayland.

However, SurfaceItemWayland::freeze() doesn't unset m_surface, so the SurfacePixmapWayland keeps updating the buffer even after the surface is unmapped. This results in some closed windows losing their contents when playing a window closing animation.

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