plugins/screenshot: Cache screenshot attributes

When a QPromise reports results, it's not necessary that the
QFutureWatcher is going to report it immediately. That can happen at
some point in the future, which is okay according to the QFuture api
contract.

Due to that, we cannot assume that the stored Output and EffectWindow
objects pointers are valid when the QFutureWatcher::finished is emitted.

src/plugins/screenshot/screenshotdbusinterface2.cpp

@@ -176,7 +176,7 @@ public:
     QVariantMap attributes() const override;
 
 private:
-    Output *m_screen;
+    QString m_name;
 };
 
 class ScreenShotSourceArea2 : public ScreenShotSource2
@@ -197,7 +197,7 @@ public:
     QVariantMap attributes() const override;
 
 private:
-    EffectWindow *m_window;
+    QUuid m_internalId;
 };
 
 class ScreenShotSinkPipe2 : public QObject
@@ -248,14 +248,14 @@ ScreenShotSourceScreen2::ScreenShotSourceScreen2(ScreenShotEffect *effect,
                                                  Output *screen,
                                                  ScreenShotFlags flags)
     : ScreenShotSource2(effect->scheduleScreenShot(screen, flags))
-    , m_screen(screen)
+    , m_name(screen->name())
 {
 }
 
 QVariantMap ScreenShotSourceScreen2::attributes() const
 {
     return QVariantMap{
-        {QStringLiteral("screen"), m_screen->name()},
+        {QStringLiteral("screen"), m_name},
     };
 }
 
@@ -270,14 +270,14 @@ ScreenShotSourceWindow2::ScreenShotSourceWindow2(ScreenShotEffect *effect,
                                                  EffectWindow *window,
                                                  ScreenShotFlags flags)
     : ScreenShotSource2(effect->scheduleScreenShot(window, flags))
-    , m_window(window)
+    , m_internalId(window->internalId())
 {
 }
 
 QVariantMap ScreenShotSourceWindow2::attributes() const
 {
     return QVariantMap{
-        {QStringLiteral("windowId"), m_window->internalId().toString()},
+        {QStringLiteral("windowId"), m_internalId.toString()},
     };
 }