Add a SurfaceInterface *WindowPixmap::surface() const

Convenience method to get the Surface for a WindowPixmap which is either
the toplevel's surface or a SubSurface.

scene.cpp

@@ -1082,6 +1082,15 @@ void WindowPixmap::updateBuffer()
     }
 }
 
+KWayland::Server::SurfaceInterface *WindowPixmap::surface() const
+{
+    if (!m_subSurface.isNull()) {
+        return m_subSurface->surface().data();
+    } else {
+        return toplevel()->surface();
+    }
+}
+
 //****************************************
 // Scene::EffectFrame
 //****************************************

scene.h

@@ -414,6 +414,11 @@ public:
         return m_subSurface;
     }
 
+    /**
+     * @returns the surface this WindowPixmap references, might be @c null.
+     **/
+    KWayland::Server::SurfaceInterface *surface() const;
+
 protected:
     explicit WindowPixmap(Scene::Window *window);
     explicit WindowPixmap(const QPointer<KWayland::Server::SubSurfaceInterface> &subSurface, WindowPixmap *parent);