The compositor will automatically unredirect the top most window which
is fully visible on screen. When unredirecting windows, it also shapes
the compositor overlay window (COW) so that other redirected windows
still shows correctly.

The function `get_top_visible_window_actor()` however will simply walks
down the window list, so if a window is placed on a layer above and
unredirected, then iconified by the client, it will still be picked up
by `get_top_visible_window_actor()` and he compositor will reckon it's
still unredirected while not in a visible state anymore, thus leaving a
black area on screen.

Make sure we skip the windows not known to the compositor while picking
the top visible window actor to avoid this issue.

Closes: #306