API review: pass QImage as const-ref also in signal arguments

Pick-to: 6.10
Change-Id: I8e47aa0facd73d0ef099836d649cc1770dcc8b0e
Reviewed-by: Sami Varanka <sami.varanka@qt.io>

tests/manual/renderslicetoimage/surfacegraphmodifier.h

@@ -22,7 +22,7 @@ public:
     QImage *renderSliceToImage(QtGraphs3D::SliceCaptureType sliceType, int requestedIndex);
 
 Q_SIGNALS:
-    void updateSliceImage(QImage image);
+    void updateSliceImage(const QImage &image);
 
 private:
     void fillSqrtSinProxy();