Replace auto &&each by const auto &each.

src/grid_icosphere.cc

@@ -119,7 +119,7 @@ subdivide(VertexList &vertices, TriangleList triangles)
   TriangleList result;
   Index mid[3];
 
-  for (auto &&each : triangles)
+  for (const auto &each : triangles)
     {
       for (int edge = 0; edge < 3; ++edge)
         mid[edge] = vertexForEdge(lookup, vertices, each[edge], each[(edge + 1) % 3]);