Windows: Panic "already borrowed" when reentering wnd_proc due to mouse move or timer

[helgoboss]

Reentering wnd_proc can happen, e.g. when opening the system web browser in an event handler via webbrowser crate.

Originally I made PR #130 for this issue, but since PR #136 has been merged, there's a more elegant way to fix the issue. I will file a new PR.