[libkwineffects] Add windowId to EffectWindow

My use case is integrating with a system that tracks window properties based on the windowId. This allows the effect to do window-specific things.

autotests/test_window_paint_data.cpp

@@ -253,6 +253,9 @@ public:
     pid_t pid() const override {
         return 0;
     }
+    qlonglong windowId() const override {
+        return 0;
+    }
 
 private:
     qreal m_opacity = 1.0;

effects.cpp

@@ -1828,6 +1828,7 @@ TOPLEVEL_HELPER(KWaylandServer::SurfaceInterface *, surface, surface)
 TOPLEVEL_HELPER(bool, isPopupWindow, isPopupWindow)
 TOPLEVEL_HELPER(bool, isOutline, isOutline)
 TOPLEVEL_HELPER(pid_t, pid, pid)
+TOPLEVEL_HELPER(qlonglong, windowId, window)
 
 #undef TOPLEVEL_HELPER
 

effects.h

@@ -454,6 +454,7 @@ public:
     bool isX11Client() const override;
 
     pid_t pid() const override;
+    qlonglong windowId() const override;
 
     QRect decorationInnerRect() const override;
     QByteArray readProperty(long atom, long type, int format) const override;

libkwineffects/kwineffects.h

@@ -2438,6 +2438,11 @@ public:
      */
     virtual pid_t pid() const = 0;
 
+    /**
+     * @since 5.21
+     */
+    virtual qlonglong windowId() const = 0;
+
     /**
      * Can be used to by effects to store arbitrary data in the EffectWindow.
      *