Add developer flag for printer agents (#15110)

src/libslic3r/AppConfig.cpp

@@ -626,6 +626,12 @@ void AppConfig::set_defaults()
         set_bool("window_buttons_on_left", false);
 #endif
 
+    if (get("use_printer_agents").empty())
+    {
+        // false = legacy behavior using print hosts
+        set_bool("use_printer_agents", false);
+    }
+
     // Remove legacy window positions/sizes
     erase("app", "main_frame_maximized");
     erase("app", "main_frame_pos");