Fix multi-tool standby heaters by turning off unused extruders (#15849)

With ooze prevention on, an extruder that has no layers left is set to 0 °C when it is parked, instead of its standby temperature. On IMEX printers where several filaments share one heater, the heater is only turned off once every filament on it is done.

src/libslic3r/GCode/ToolOrdering.cpp

@@ -1276,6 +1276,13 @@ void ToolOrdering::collect_extruder_statistics(bool prime_multi_material)
         sort_remove_duplicates(m_all_printing_extruders);
     }
 
+    m_last_layer_per_extruder.clear();
+    for (const LayerTools& lt : m_layer_tools) {
+        for (unsigned int ext : lt.extruders) {
+            m_last_layer_per_extruder[ext] = lt.print_z;
+        }
+    }
+
     if (prime_multi_material && ! m_all_printing_extruders.empty()) {
         // Reorder m_all_printing_extruders in the sequence they will be primed, the last one will be m_first_printing_extruder.
         // Then set m_first_printing_extruder to the 1st extruder primed.
@@ -3664,5 +3671,11 @@ int WipingExtrusions::get_support_interface_extruder_overrides(const PrintObject
     return -1;
 }
 
-
+bool ToolOrdering::is_last_extrusion_layer(coordf_t print_z, unsigned int extruder_id) const
+{
+    auto it = m_last_layer_per_extruder.find(extruder_id);
+    if (it == m_last_layer_per_extruder.end())
+        return true;
+    return print_z >= it->second - EPSILON;
+}
 } // namespace Slic3r