message: Clean g.WindowsById storage on shutdown.
diff: @@ -2464,6 +2464,7 @@ void ImGui::Shutdown()
  g.WindowsSortBuffer.clear();
  g.CurrentWindow = NULL;
  g.CurrentWindowStack.clear();
  + g.WindowsById.Clear();
  g.NavWindow = NULL;
  g.HoveredWindow = NULL;
  g.HoveredRootWindow = NULL;
  