Send all global_inherits_stack to engine

Only the settings for which global_inherits_stack is set should be sent to the engine.

Contributes to issue CURA-2011.

plugins/CuraEngineBackend/Cura.proto

@@ -13,6 +13,7 @@ message Slice
     repeated ObjectList object_lists = 1; // The meshgroups to be printed one after another
     SettingList global_settings = 2; // The global settings used for the whole print job
     repeated Extruder extruders = 3; // The settings sent to each extruder object
+    repeated SettingExtruder global_inherits_stack = 4; //From which stack the setting would inherit if not defined in a stack.
 }
 
 message Extruder
@@ -108,8 +109,14 @@ message Setting {
     bytes value = 2; // The value of the setting
 }
 
+message SettingExtruder {
+    string name = 1; //The setting key.
+
+    int32 extruder = 2; //From which extruder stack the setting should inherit.
+}
+
 message GCodePrefix {
-    bytes data = 2; // Header string to be prenpended before the rest of the gcode sent from the engine
+    bytes data = 2; //Header string to be prepended before the rest of the g-code sent from the engine.
 }
 
 message SlicingFinished {

plugins/CuraEngineBackend/StartSliceJob.py

@@ -134,6 +134,7 @@ class StartSliceJob(Job):
                 return
 
             self._buildGlobalSettingsMessage(stack)
+            self._buildGlobalInheritsStackMessage(stack)
 
             for extruder_stack in cura.Settings.ExtruderManager.getInstance().getMachineExtruders(stack.getId()):
                 self._buildExtruderMessage(extruder_stack)
@@ -216,6 +217,21 @@ class StartSliceJob(Job):
             else:
                 setting_message.value = str(value).encode("utf-8")
 
+    ##  Sends for some settings which extruder they should fallback to if not
+    #   set.
+    #
+    #   This is only set for settings that have the global_inherits_stack
+    #   property.
+    #
+    #   \param stack The global stack with all settings, from which to read the
+    #   global_inherits_stack property.
+    def _buildGlobalInheritsStackMessage(self, stack):
+        for key in stack.getAllKeys():
+            if stack.hasProperty(key, "global_inherits_stack"):
+                setting_extruder = self._slice_message.addRepeatedMessage("global_inherits_stack")
+                setting_extruder.name = key
+                setting_extruder.extruder = int(stack.getProperty(key, "global_inherits_stack"))
+
     ##  Check if a node has per object settings and ensure that they are set correctly in the message
     #   \param node \type{SceneNode} Node to check.
     #   \param message object_lists message to put the per object settings in