Improve performance of mesh-forced assemblies.

src/groupmesh.cpp

@@ -357,10 +357,14 @@ void Group::GenerateShellAndMesh() {
         // and corresponding slowdown.
         outm.RemoveDegenerateTriangles();
 
+        if(srcg->meshCombine != CombineAs::ASSEMBLE) {
             // And make sure that the output mesh is vertex-to-vertex.
             SKdNode *root = SKdNode::From(&outm);
             root->SnapToMesh(&outm);
             root->MakeMeshInto(&runningMesh);
+        } else {
+            runningMesh.MakeFromCopyOf(&outm);
+        }
 
         outm.Clear();
         thism.Clear();