Keep the First Value When a Per-Filament Variant Option Is Too Short (#15639)

update_values_to_printer_extruders_for_multiple_filaments picks each
filament's value from the flattened (filament x variant) columns of every
per-filament variant option. When a column index fell past the end of the
option's values, it skipped that filament and left the zero the output
vector was created with.

The GUI always hands this function full columns, but the CLI does not:

- a CLI override of a single value, such as --nozzle-temperature=211 on a
  four-filament project, came out as 211,0,0,0, so three filaments would
  print at 0 C;
- loading fewer filament presets than the project has filaments left the
  remaining filaments' columns missing, so filament_cooling_before_tower
  came out as 10,10,0,0 and filament_ramming_volumetric_speed as -1,-1,0,0.

An out-of-range column now keeps the option's first value, the fallback
get_at() and the sibling gather step already use. The seven per-type copies
of the loop are replaced by that same gather_option_values helper, moved
above the function; it now takes its caller's name for its log lines. An
empty option, which has no first value, is given one registered default per
filament first; it used to be replaced with zeros.

On a partial load a filament whose preset was not loaded takes the first
filament's value rather than its own preset's, which the CLI does not load;
for the options seen in practice those agree.

tests/libslic3r/test_config_variant_expansion.cpp

@@ -484,6 +484,34 @@ TEST_CASE("update_values_to_printer_extruders_for_multiple_filaments resolves pe
         REQUIRE(config.option<ConfigOptionFloats>("filament_max_volumetric_speed")->values == std::vector<double>({12., 21.}));
         REQUIRE(config.option<ConfigOptionInts>("filament_self_index")->values == std::vector<int>({1, 2}));
     }
+
+    SECTION("a variant option shorter than the filament slots keeps its first value instead of zero") {
+        DynamicPrintConfig config;
+        config.option<ConfigOptionEnumsGeneric>("extruder_type", true)->values = {etDirectDrive, etDirectDrive};
+        config.option<ConfigOptionEnumsGeneric>("nozzle_volume_type", true)->values = {nvtStandard, nvtHighFlow};
+        config.option<ConfigOptionStrings>("extruder_variant_list", true)->values = {"Direct Drive Standard,Direct Drive High Flow",
+                                                                                     "Direct Drive Standard,Direct Drive High Flow"};
+        make_filament_arrays(config);
+        config.option<ConfigOptionInts>("filament_map", true)->values = {1, 2};
+        // no loaded preset carries the key, so only its single registered default is present
+        config.option<ConfigOptionFloatsNullable>("filament_cooling_before_tower", true)->values = {10.};
+        // only the first filament's two variant columns were loaded
+        config.option<ConfigOptionFloatsNullable>("filament_ramming_volumetric_speed", true)->values = {-1., -2.};
+
+        std::vector<std::vector<NozzleVolumeType>> nozzle_volume_types;
+        int extruder_count = 2;
+        int count = config.get_extruder_nozzle_volume_count(extruder_count, nozzle_volume_types);
+
+        config.update_values_to_printer_extruders_for_multiple_filaments(config, extruder_count, count, filament_keys,
+            "filament_self_index", "filament_extruder_variant");
+
+        // filament 2 resolves to column 3 (its extruder's High Flow column), past the end of both vectors
+        REQUIRE_THAT(config.option<ConfigOptionFloatsNullable>("filament_cooling_before_tower")->values,
+                     Catch::Matchers::Approx(std::vector<double>({10., 10.})));
+        REQUIRE_THAT(config.option<ConfigOptionFloatsNullable>("filament_ramming_volumetric_speed")->values,
+                     Catch::Matchers::Approx(std::vector<double>({-1., -1.})));
+        REQUIRE(config.option<ConfigOptionFloats>("filament_max_volumetric_speed")->values == std::vector<double>({12., 21.}));
+    }
 }
 
 // update_values_from_multi_to_multi_2 walks the DESTINATION PRINTER's variant list while writing