Add 3 new gcode settings

- Add bool to set if the start_gcode needs to be the first gcode run.  Requires `machine_start_gcode_first` be added to the machine UI as a checkbox
- Add a prestart_gcode to extruders, this will allow use set temperatures before the change call so we can preheat the next extruder.  Requires `machine_extruder_prestart_code` text box be added to the extruder UI page.
- Add a time duration to calculate time required to change extruders. Requires `machine_extruder_change_duration` float box to be added to the exturder UI page.
- Fix a bug in the export code that was checking num_extruders instead of used_extruders, this was adding a tool change on start to T0 when only one extruder is used which we do not want, with one extruder it should output like normal and let the user choose the extruder they want to use.

src/gcodeExport.cpp

@@ -682,6 +682,11 @@ bool GCodeExport::initializeExtruderTrains(const SliceDataStorage& storage, cons
 
     writeComment("Generated with Cura_SteamEngine " CURA_ENGINE_VERSION);
 
+    if (mesh_group_settings.get<bool>("machine_start_gcode_first"))
+    {
+        writeCode(mesh_group_settings.get<std::string>("machine_start_gcode").c_str());
+    }
+
     if (getFlavor() == EGCodeFlavor::GRIFFIN)
     {
         std::ostringstream tmp;
@@ -693,8 +698,11 @@ bool GCodeExport::initializeExtruderTrains(const SliceDataStorage& storage, cons
         processInitialLayerTemperature(storage, start_extruder_nr);
     }
 
+    if (!mesh_group_settings.get<bool>("machine_start_gcode_first"))
+    {
+        writeCode(mesh_group_settings.get<std::string>("machine_start_gcode").c_str());
+    }
     writeExtrusionMode(false); // ensure absolute extrusion mode is set before the start gcode
-    writeCode(mesh_group_settings.get<std::string>("machine_start_gcode").c_str());
 
     // in case of shared nozzle assume that the machine-start gcode reset the extruders as per machine description
     if (Application::getInstance().current_slice_->scene.settings.get<bool>("machine_extruders_share_nozzle"))
@@ -767,6 +775,8 @@ void GCodeExport::processInitialLayerTemperature(const SliceDataStorage& storage
     const bool material_print_temp_prepend = scene.current_mesh_group->settings.get<bool>("material_print_temp_prepend");
     const bool material_print_temp_wait = scene.current_mesh_group->settings.get<bool>("material_print_temp_wait");
     bool wait_start_extruder = false;
+    std::vector<bool> extruders_used = storage.getExtrudersUsed();
+    size_t used_extruders = std::count(extruders_used.begin(), extruders_used.end(), true);
 
     switch (getFlavor())
     {
@@ -776,7 +786,7 @@ void GCodeExport::processInitialLayerTemperature(const SliceDataStorage& storage
         wait_start_extruder = true;
         break;
     default:
-        if (num_extruders > 1 || getFlavor() == EGCodeFlavor::REPRAP)
+        if (used_extruders > 1 || getFlavor() == EGCodeFlavor::REPRAP)
         {
             std::ostringstream tmp;
             tmp << "T" << start_extruder_nr;
@@ -794,7 +804,6 @@ void GCodeExport::processInitialLayerTemperature(const SliceDataStorage& storage
     };
 
     std::vector<ExtruderInitialize> all_extruders;
-    std::vector<bool> extruders_used = storage.getExtrudersUsed();
     for (size_t extruder_nr = 0; extruder_nr < extruders_used.size(); ++extruder_nr)
     {
         if (extruders_used[extruder_nr])
@@ -1267,6 +1276,29 @@ void GCodeExport::writeZhopEnd(Velocity speed /*= 0*/)
 
 void GCodeExport::startExtruder(const size_t new_extruder)
 {
+    const auto extruder_settings = Application::getInstance().current_slice_->scene.extruders[new_extruder].settings_;
+    const auto prestart_code = extruder_settings.get<std::string>("machine_extruder_prestart_code");
+    const auto start_code = extruder_settings.get<std::string>("machine_extruder_start_code");
+    const auto start_code_duration = extruder_settings.get<Duration>("machine_extruder_start_code_duration");
+    const auto extruder_change_duration = extruder_settings.get<Duration>("machine_extruder_change_duration");
+
+    // Be nice to be able to calculate the extruder change time verses time
+    // to heat and run this so it's run before the change call. **Future note**
+    if (! prestart_code.empty())
+    {
+        if (relative_extrusion_)
+        {
+            writeExtrusionMode(false); // ensure absolute extrusion mode is set before the prestart gcode
+        }
+
+        writeCode(prestart_code.c_str());
+
+        if (relative_extrusion_)
+        {
+            writeExtrusionMode(true); // restore relative extrusion mode
+        }
+    }
+
     extruder_attr_[new_extruder].is_used_ = true;
     if (new_extruder != current_extruder_) // wouldn't be the case on the very first extruder start if it's extruder 0
     {
@@ -1278,15 +1310,16 @@ void GCodeExport::startExtruder(const size_t new_extruder)
         {
             *output_stream_ << "T" << new_extruder << new_line_;
         }
+        // Only add time is we are actually changing extruders
+        estimate_calculator_.addTime(extruder_change_duration);
     }
 
+    estimate_calculator_.addTime(start_code_duration);
     current_extruder_ = new_extruder;
 
     assert(getCurrentExtrudedVolume() == 0.0 && "Just after an extruder switch we haven't extruded anything yet!");
     resetExtrusionValue(); // zero the E value on the new extruder, just to be sure
 
-    const auto extruder_settings = Application::getInstance().current_slice_->scene.extruders[new_extruder].settings_;
-    const auto start_code = extruder_settings.get<std::string>("machine_extruder_start_code");
     if (! start_code.empty())
     {
         if (relative_extrusion_)
@@ -1302,9 +1335,6 @@ void GCodeExport::startExtruder(const size_t new_extruder)
         }
     }
 
-    const auto start_code_duration = extruder_settings.get<Duration>("machine_extruder_start_code_duration");
-    estimate_calculator_.addTime(start_code_duration);
-
     Application::getInstance().communication_->setExtruderForSend(Application::getInstance().current_slice_->scene.extruders[new_extruder]);
     Application::getInstance().communication_->sendCurrentPosition(getPositionXY());
 

src/sliceDataStorage.cpp

@@ -398,6 +398,12 @@ std::vector<bool> SliceDataStorage::getExtrudersUsed() const
     std::vector<bool> ret;
     ret.resize(Application::getInstance().current_slice_->scene.extruders.size(), false);
 
+    // set all the false to start, we set them to true if used
+    for (size_t extruder_nr = 0; extruder_nr < Application::getInstance().current_slice_->scene.extruders.size(); extruder_nr++)
+    {
+        ret[extruder_nr] = false;
+    }
+
     const Settings& mesh_group_settings = Application::getInstance().current_slice_->scene.current_mesh_group->settings;
     const EPlatformAdhesion adhesion_type = mesh_group_settings.get<EPlatformAdhesion>("adhesion_type");
     if (adhesion_type == EPlatformAdhesion::SKIRT || adhesion_type == EPlatformAdhesion::BRIM)