overview: Set searchEntry offscreen-redirected always

overview: Set searchEntry offscreen-redirected always

This corrects weird-looking blending visible as it fades out when the
overview closes. Previously the entry's dark background would drown out
the text as it fades out, but now they maintain a consistent contrast ratio
during the fade.

There's no noticeable change in performance, but in theory it should be
faster as text entries don't change at full frame rate. So stage redraws
will usually have a cached searchEntry drawn and require less effort.
Though the main purpose here is to correct the appearance.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/778