Add OpenMP parallel execution createLayerParts (CURA-3331)

src/layerPart.cpp

@@ -3,6 +3,7 @@
 #include "layerPart.h"
 #include "settings/settings.h"
 #include "progress/Progress.h"
+#include "multithreadOpenMP.h"
 
 #include "utils/SVG.h" // debug output
 
@@ -44,7 +45,10 @@ void createLayerWithParts(SliceLayer& storageLayer, SlicerLayer* layer, bool uni
 }
 void createLayerParts(SliceMeshStorage& mesh, Slicer* slicer, bool union_layers, bool union_all_remove_holes)
 {
-    for(unsigned int layer_nr = 0; layer_nr < slicer->layers.size(); layer_nr++)
+    const auto total_layers = slicer->layers.size();
+    assert(mesh.layers.size() == total_layers);
+#pragma omp parallel for default(none) shared(mesh,slicer) firstprivate(union_layers,union_all_remove_holes) schedule(dynamic)
+    for(unsigned int layer_nr = 0; layer_nr < total_layers; layer_nr++)
     {
         mesh.layers[layer_nr].sliceZ = slicer->layers[layer_nr].z;
         mesh.layers[layer_nr].printZ = slicer->layers[layer_nr].z;