[regression] Newly open windows restacking relative to each other can end up above panels

[Sam Spilsbury]

It is possible for windows created in time right next to each other that need to restack relative to each other to have their restack operations fail and end up on top of the shell. This is because we search for the restacking target before configuring the window and reject the operation if it was invalid. However, that code to check if the operation was valid was broken because it didn't use the most up to date server state. This would result in warnings such as:

Warn - Attempted to restack window relative to ... which is not a window that compiz owns

Those windows would fail to restack, and if just created, end up on top.

This can cause gnome-terminal and mumble to open above the dash, for instance.