compositor: Skip windows not visible to the compositor
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 show 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 (closed)