If we are previewing hidden windows, we might be previewing them in a context
like a thumbnail view of a workspace where we care about positioning. So, instead
of waiting until the window is first actually shown to place it, if
live_hidden_previews is set, place the window window when we first compute its
visibility, even if we don't end up showing it.
https://bugzilla.gnome.org/show_bug.cgi?id=641309