Lay flat gizmo improvements - merge adjacent faces, compute and cache convex hull for entire ModelObject, refresh when moved, etc.

xs/src/libslic3r/TriangleMesh.cpp

@@ -728,6 +728,7 @@ TriangleMesh TriangleMesh::convex_hull_3d() const
 
     TriangleMesh output_mesh(det_vertices, facets);
     output_mesh.repair();
+    output_mesh.require_shared_vertices();
     return output_mesh;
 }