pass background to core

GUI/Model/Device/InstrumentItems.cpp

@@ -441,7 +441,13 @@ ISimulation* SpecularInstrumentItem::createSimulation(const MultiLayer& sample)
 
     std::unique_ptr<Scale> axis = axis_item->itemToAxis(Units::deg, *converted_axis);
     std::unique_ptr<IBeamScan> scan = createScan(*axis);
-    return new SpecularSimulation(*scan, sample);
+
+    auto* result = new SpecularSimulation(*scan, sample);
+
+    if (const auto background = backgroundItem()->createBackground())
+        result->setBackground(*background);
+
+    return result;
 }
 
 void SpecularInstrumentItem::writeTo(QXmlStreamWriter* w) const