[kcm/tabbox] Add missing property to mock ThumbnailItem

The TabBox preview uses a mock WindowThumbnailItem mimicking the
real item, but it was missing the `sourceSize` property, preventing
some effects to be previewed if they use it.

src/kcmkwin/kwintabbox/thumbnailitem.cpp

@@ -100,6 +100,7 @@ WindowThumbnailItem::WindowThumbnailItem(QQuickItem* parent)
     , m_clipToItem(nullptr)
     , m_brightness(1.0)
     , m_saturation(1.0)
+    , m_sourceSize(QSize())
 {
     setFlag(ItemHasContents);
 }
@@ -185,6 +186,11 @@ qreal WindowThumbnailItem::saturation() const
     return m_saturation;
 }
 
+QSize WindowThumbnailItem::sourceSize() const
+{
+    return m_sourceSize;
+}
+
 void WindowThumbnailItem::setBrightness(qreal brightness)
 {
     if (m_brightness == brightness) {
@@ -205,4 +211,14 @@ void WindowThumbnailItem::setSaturation(qreal saturation)
     Q_EMIT saturationChanged();
 }
 
+void WindowThumbnailItem::setSourceSize(const QSize &size)
+{
+    if (m_sourceSize == size) {
+        return;
+    }
+    m_sourceSize = size;
+    update();
+    Q_EMIT sourceSizeChanged();
+}
+
 } // namespace KWin

src/kcmkwin/kwintabbox/thumbnailitem.h

@@ -54,6 +54,7 @@ class WindowThumbnailItem : public QQuickItem
     Q_PROPERTY(QQuickItem *clipTo READ clipTo WRITE setClipTo NOTIFY clipToChanged)
     Q_PROPERTY(qreal brightness READ brightness WRITE setBrightness NOTIFY brightnessChanged)
     Q_PROPERTY(qreal saturation READ saturation WRITE setSaturation NOTIFY saturationChanged)
+    Q_PROPERTY(QSize sourceSize READ sourceSize WRITE setSourceSize NOTIFY sourceSizeChanged)
 public:
     explicit WindowThumbnailItem(QQuickItem *parent = nullptr);
     ~WindowThumbnailItem() override;
@@ -66,10 +67,12 @@ public:
     }
     qreal brightness() const;
     qreal saturation() const;
+    QSize sourceSize() const;
     void setWId(qulonglong wId);
     void setClipTo(QQuickItem *clip);
     void setBrightness(qreal brightness);
     void setSaturation(qreal saturation);
+    void setSourceSize(const QSize &size);
     QSGNode *updatePaintNode(QSGNode *oldNode, UpdatePaintNodeData *updatePaintNodeData) override;
 
     enum Thumbnail {
@@ -83,6 +86,7 @@ Q_SIGNALS:
     void clipToChanged();
     void brightnessChanged();
     void saturationChanged();
+    void sourceSizeChanged();
 private:
     void findImage();
     qulonglong m_wId;
@@ -90,6 +94,7 @@ private:
     QQuickItem *m_clipToItem;
     qreal m_brightness;
     qreal m_saturation;
+    QSize m_sourceSize;
 };
 
 } // KWin