It's only obliquely documented, but -[NSView addSubview:positioned:relativeTo:]
will remove the view from its old superview if it's changing superviews.  If
it's just changing z-order within its current superview, it won't.

This avoids some flicker of OpenGL surfaces being removed and re-added.
Signed-off-by: Ken Thomases <ken@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>