Write hollow flag to SL1 files if any object is hollowed.

src/libslic3r/Format/SL1.cpp

@@ -371,6 +371,13 @@ void fill_iniconf(ConfMap &m, const SLAPrint &print)
     m["numSlow"]      = std::to_string(stats.slow_layers_count);
     m["numFast"]      = std::to_string(stats.fast_layers_count);
     m["printTime"]    = std::to_string(stats.estimated_print_time);
+
+    bool hollow_en = false;
+    auto it = print.objects().begin();
+    while (!hollow_en && it != print.objects().end())
+        hollow_en = hollow_en || (*it++)->config().hollowing_enable.getBool();
+
+    m["hollow"] = hollow_en ? "1" : "0";
     
     m["action"] = "print";
 }