wayland/tablet_v2: Keep also the pad surface in a QPointer

This way we make sure that we don't explode if for some reason the
surface is destroyed (e.g. it's closed).
This will make it work exactly like the other references to
SurfaceInterface.

BUG: 456817

src/wayland/tablet_v2_interface.cpp

@@ -542,7 +542,7 @@ public:
     QVector<TabletPadStripV2Interface *> m_strips;
     TabletPadGroupV2Interface *const m_padGroup;
     TabletSeatV2Interface *m_seat = nullptr;
-    SurfaceInterface *m_currentSurface = nullptr;
+    QPointer<SurfaceInterface> m_currentSurface;
     Display *const m_display;
 };