Don't silently let windows that we couldn't find a sibling below for sit
at the top of the stack (LP #936675)
If we couldn't find a sibling for the window upon initial map, then it means
that it needs to be placed at the bottom of the stack, since there could be
windows above it which should not be below the newly created window.
Fixes a case where on startup, nautilus could be slow to load and new windows
would be created before nautilus was, and those would go above panels incorrectly.
Merge lp:~smspillaz/compiz-core/compiz-core.fix_936675 into lp:compiz-core