Freeze break: mutter: redrawing shadows on focus change

[Owen Taylor <otaylor redhat com>]

[ Continuing on my trend of pushing for patches to fix some
  ugly visual nits in the core features of GNOME Shell ]

If focus changes without restacking windows:

 - Because focus is lost to a shell notification
 - Because focus is lost to a window in a different layer (such
   as an always-on-top window)

Then shadows are misdrawn in gnome-shell, as seen in the screenshot in:

 https://bugzilla.gnome.org/show_bug.cgi?id=636904

This is only a visual glitch, but it does break the illusion if you
get half a shadow when interacting with the message tray. 

Dan's patch to fix is somewhat complex, but I've banged on it pretty
extensively and it makes good sense to me as a logical and consistent
way to do things.

(Note that it also won't cause crashes even if
there are bugs in updating window->attached_focus_window since
window->attached_focus_window is never dereferenced. Worst case scenario
would be a bug resulting in a window "sticking" drawing as focused
when it isn't are it's unfocused.)

- Owen