restore projection live display (#883)

GUI/View/Canvas/MaskEditorCanvas.cpp

@@ -136,9 +136,10 @@ void MaskEditorCanvas::onPositionChanged(double x, double y)
 {
     if (m_live_projection) {
         if (m_current_activity == Canvas2DMode::HORIZONTAL_PRJN)
-            dynamic_cast<HorizontalLineItem*>(m_live_projection.get())->setPos(y);
+            m_live_projection->setPos(y);
         else if (m_current_activity == Canvas2DMode::VERTICAL_PRJN)
-            dynamic_cast<VerticalLineItem*>(m_live_projection.get())->setPos(x);
+            m_live_projection->setPos(x);
+        emit m_data_item->projectionPositionChanged(m_live_projection.get());
     }
 }