Saveload: Add save flag for if this is a scenario save

src/saveload/saveload.cpp

@@ -3312,6 +3312,11 @@ bool IsNetworkServerSave()
 	return _sl.save_flags & SMF_NET_SERVER;
 }
 
+bool IsScenarioSave()
+{
+	return _sl.save_flags & SMF_SCENARIO;
+}
+
 struct ThreadedLoadFilter : LoadFilter {
 	static const size_t BUFFER_COUNT = 4;