Lay flat - icons and invalidation improvement

xs/src/libslic3r/TriangleMesh.cpp

@@ -732,6 +732,11 @@ TriangleMesh TriangleMesh::convex_hull_3d() const
     return output_mesh;
 }
 
+const float* TriangleMesh::first_vertex() const
+{
+    return stl.facet_start ? &stl.facet_start->vertex[0].x : nullptr;
+}
+
 void
 TriangleMesh::require_shared_vertices()
 {

xs/src/libslic3r/TriangleMesh.hpp

@@ -53,6 +53,7 @@ public:
     TriangleMeshPtrs split() const;
     void merge(const TriangleMesh &mesh);
     ExPolygons horizontal_projection() const;
+    const float* first_vertex() const;
     Polygon convex_hull();
     BoundingBoxf3 bounding_box() const;
     // Returns the bbox of this TriangleMesh transformed by the given matrix