Store surface object in tablet cursor using QPointer

The SurfaceInterface can get destroyed and TabletCursorV2 may return a
dangling pointer.

BUG: 446255

src/wayland/tablet_v2_interface.cpp

@@ -91,7 +91,7 @@ public:
     TabletCursorV2 *const q;
 
     quint32 m_serial = 0;
-    SurfaceInterface *m_surface = nullptr;
+    QPointer<SurfaceInterface> m_surface;
     QPoint m_hotspot;
 };