cursor: don't un-minimize previewed window while window switching

cursor: don't un-minimize previewed window while window switching

This commit restores the check removed in 7a6ecca.

Without the check, if followMouse="yes" and
followMouseRequiresMovement="no", osd_update() => cursor_update_focus() =>
desktop_focus_view() unexpectedly un-minimizes the window on cursor even
when the window is just a preview of window switcher. This caused some
strange behavior that a minimized window selected with window switcher is
immediately hidden after finishing window switching.