message: demo: Add coverage test for IndexAdapter out
  We only tested IndexAdapter input before.
diff: @@ -301,11 +301,15 @@ static void customAllocator()
  meshopt_optimizeVertexFetch(vb, ib, 3, vb, 3, 12);
  assert(allocCount == 3 && freeCount == 3);
  + // ... plus one for IndexAdapter
  + meshopt_optimizeVertexFetch(vb, ibs, 3, vb, 3, 12);
  + assert(allocCount == 6 && freeCount == 6);
  +
  meshopt_setAllocator(operator new, operator delete);
  // customAlloc & customFree should not get called anymore
  meshopt_optimizeVertexFetch(vb, ib, 3, vb, 3, 12);
  - assert(allocCount == 3 && freeCount == 3);
  + assert(allocCount == 6 && freeCount == 6);
  }
  static void emptyMesh()
  