message: demo: Change encodeIndex to use meshopt_Buffer
  This makes decode timings more truthful since they no longer include
  memset (clearing std::vector contents during resize).
diff: @@ -389,7 +389,8 @@ void encodeIndex(const Mesh& mesh)
  double middle = timestamp();
  - std::vector<unsigned int> result(mesh.indices.size());
  + // using meshopt_Buffer instead of std::vector to avoid memset overhead
  + meshopt_Buffer<unsigned int> result(mesh.indices.size());
  int res = meshopt_decodeIndexBuffer(&result[0], mesh.indices.size(), &buffer[0], buffer.size());
  assert(res == 0);
  (void)res;
  @@ -411,7 +412,7 @@ void encodeIndex(const Mesh& mesh)
  double(csize * 8) / double(mesh.indices.size() / 3),
  (middle - start) * 1000,
  (end - middle) * 1000,
  - (double(result.size() * 4) / (1 << 30)) / (end - middle));
  + (double(result.size * 4) / (1 << 30)) / (end - middle));
  }
  struct PackedVertex
  