Keep Auto Drop off for projects saved by 2.4.x (#16633)

2.4.0 to 2.4.2 wrote auto_drop on each build item, and 2.4.x objects
carry no auto_drop attribute in model_settings.config. The importer only
read the object attribute and treated a missing one as on, so floating
objects were dropped to the bed on load and the flag was rewritten as on
on the next save.

Record object ids with an item auto_drop="0" and use that as the fallback
when the object has no attribute. The attribute's presence is now tracked
so the fallback applies only when it is absent. Any item set to off turns
auto drop off for the object, since the flag is shared by all instances.

tests/libslic3r/test_3mf.cpp

@@ -505,6 +505,28 @@ TEST_CASE("A project whose components reference themselves fails to load", "[3mf
     REQUIRE_FALSE(loaded);
 }
 
+TEST_CASE("A project in the 2.4.x layout keeps auto drop off from its build item", "[3mf][Regression]")
+{
+    // 2.4.x wrote auto_drop on the build item and left it off the object, so the object is re-laid out here.
+    ScopedTemporaryFile temp(".3mf");
+    {
+        Model        model;
+        ModelObject* object = model.add_object();
+        object->add_volume(make_cube(10., 10., 10.));
+        object->add_instance();
+        object->auto_drop = false;
+        store_project(model, temp.string(), SaveStrategy::Zip64 | SaveStrategy::Silence);
+    }
+    REQUIRE(replace_in_3mf_entry(temp.string(), "model_settings.config", " auto_drop=\"0\"", ""));
+    REQUIRE(replace_in_3mf_entry(temp.string(), "3dmodel.model", "printable=\"1\"/>", "printable=\"1\" auto_drop=\"0\"/>"));
+
+    ScopedTemporaryDir backup_dir("orca_auto_drop_dst");
+    Model              model;
+    REQUIRE(load_project(temp.string(), model, backup_dir));
+    REQUIRE(model.objects.size() == 1);
+    CHECK_FALSE(model.objects.front()->auto_drop);
+}
+
 TEST_CASE("An object loads up to the component reference budget and fails past it", "[3mf][Regression]")
 {
     // The importer queues at most 100000 component references per object. Every reference besides the