scene: Fix SurfaceItemWayland::freeze()

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.

src/scene/surfaceitem_wayland.cpp

@@ -196,6 +196,8 @@ void SurfaceItemWayland::freeze()
     for (auto &[subsurface, subsurfaceItem] : m_subsurfaces) {
         subsurfaceItem->freeze();
     }
+
+    m_surface = nullptr;
 }
 
 void SurfaceItemWayland::handleColorDescriptionChanged()