message: demo: Clarify optOverdrawOnly
  Highlight the usage of single cluster and ACMR 3 to provide overdraw
  optimizer with maximum freedom.
diff: @@ -155,10 +155,11 @@ void optOverdrawOnly(Mesh& mesh)
  {
  std::vector<unsigned int> result(mesh.indices.size());
  - std::vector<unsigned int> clusters;
  - clusters.push_back(0);
  + // use single input cluster encompassing the entire mesh and worst-case ACMR so that overdraw optimizer can sort *all* triangles
  + std::vector<unsigned int> clusters(1);
  + const float kThreshold = 3.f;
  - optimizeOverdraw(&result[0], &mesh.indices[0], mesh.indices.size(), &mesh.vertices[0].px, sizeof(Vertex), mesh.vertices.size(), clusters, kCacheSize, 3.f);
  + optimizeOverdraw(&result[0], &mesh.indices[0], mesh.indices.size(), &mesh.vertices[0].px, sizeof(Vertex), mesh.vertices.size(), clusters, kCacheSize, kThreshold);
  mesh.indices.swap(result);
  }
  