Keep a Project's Changed Values on Extruder Variants It Doesn't List (#16107)

A project's listed settings are carried onto its base preset by update_non_diff_values_to_base_config, which matched variants by exact name and id. A variant the base gained after the project was saved got the base's value, while the same value in a user preset now falls back to the preset's first variant of that extruder. So an old project opened with its printer preset already modified, and saving it wrote the base's values into the 3MF.

The function now maps variants with map_variant_indices, as update_diff_values_to_child_config does: a base variant the project does not list takes the project's first variant of the same extruder. The variant lists themselves stay the base's, so a fallback never writes one variant's name over another's.

tests/libslic3r/test_preset_bundle_loading.cpp

@@ -6228,8 +6228,9 @@ TEST_CASE("A per-variant project value maps onto its base preset's variant layou
                                                                    base_finder(&base, calls));
     CHECK(config.option<ConfigOptionStrings>("print_extruder_variant")->values ==
           std::vector<std::string>{"Direct Drive Standard", "Direct Drive High Flow"});
-    // The listed key keeps the project's Standard value and takes High Flow from the base.
-    check_double_vector(config.option<ConfigOptionFloats>("outer_wall_speed")->values, {100., 300.});
+    // The listed key keeps the project's Standard value, and High Flow, which the project does not
+    // list, takes it too, as a user preset's value does.
+    check_double_vector(config.option<ConfigOptionFloats>("outer_wall_speed")->values, {100., 100.});
     check_double_vector(config.option<ConfigOptionFloats>("inner_wall_speed")->values, {250., 350.});
 }