fix: use QPointer for ImageAnalyser::m_sourceItem

Raw QQuickItem* was not tracked for destruction, causing
use-after-free if the source item was destroyed between
setSourceItem() and deferred requestUpdate() invocations.

plugin/src/Caelestia/imageanalyser.hpp

@@ -4,6 +4,7 @@
 #include <qfuture.h>
 #include <qfuturewatcher.h>
 #include <qobject.h>
+#include <qpointer.h>
 #include <qqmlintegration.h>
 
 namespace caelestia {
@@ -48,7 +49,7 @@ private:
     QFutureWatcher<AnalyseResult>* const m_futureWatcher;
 
     QString m_source;
-    QQuickItem* m_sourceItem;
+    QPointer<QQuickItem> m_sourceItem;
     int m_rescaleSize;
 
     QColor m_dominantColour;