Recalculate the start / end elements when the layer is changed instead of on render

About with larger models, about 30-40% of the time of the render was spent on this.

plugins/SimulationView/SimulationView.py

@@ -71,6 +71,8 @@ class SimulationView(CuraView):
         self._max_paths = 0
         self._current_path_num = 0
         self._minimum_path_num = 0
+        self.start_elements_index = 0
+        self.end_elements_index = 0
         self.currentLayerNumChanged.connect(self._onCurrentLayerNumChanged)
 
         self._busy = False
@@ -243,6 +245,7 @@ class SimulationView(CuraView):
                 self._minimum_layer_num = self._current_layer_num
 
             self._startUpdateTopLayers()
+            self.recalculateStartEndElements()
 
             self.currentLayerNumChanged.emit()
 
@@ -257,7 +260,7 @@ class SimulationView(CuraView):
                 self._current_layer_num = self._minimum_layer_num
 
             self._startUpdateTopLayers()
-
+            self.recalculateStartEndElements()
             self.currentLayerNumChanged.emit()
 
     def setPath(self, value: int) -> None:
@@ -359,6 +362,24 @@ class SimulationView(CuraView):
             return 0.0 # If it's still max-float, there are no measurements. Use 0 then.
         return self._min_thickness
 
+    def recalculateStartEndElements(self):
+        self.start_elements_index = 0
+        self.end_elements_index = 0
+        scene = self.getController().getScene()
+        for node in DepthFirstIterator(scene.getRoot()):  # type: ignore
+            layer_data = node.callDecoration("getLayerData")
+            if not layer_data:
+                continue
+
+            # Found a the layer data!
+            element_counts = layer_data.getElementCounts()
+            for layer in sorted(element_counts.keys()):
+                if layer == self._current_layer_num:
+                    break
+                if self._minimum_layer_num > layer:
+                    self.start_elements_index += element_counts[layer]
+                self.end_elements_index += element_counts[layer]
+
     def getMaxThickness(self) -> float:
         return self._max_thickness