compositor: Ignore offscreen windows when unredirecting

compositor: Ignore offscreen windows when unredirecting

When determining whether we should unredirect a window or not, ignore
offscreen windows, and just check the top most visible window.

Previously this was not an issue, but since 'stack-tracker: Keep
override redirect windows on top' we started sorting the UI frames
window, which is an offscreen override redirect window, on top, causing
the unredirect checking code to always check whether to unredirect the
UI frames window. This effectively disabled the compositor bypass
functionality.

https://bugzilla.gnome.org/show_bug.cgi?id=788493

Gbp-Pq: Name 0013-compositor-Ignore-offscreen-windows.patch