MetaStackTracker: optimize out unnecessary X restacking

We have a quite accurate view of the X stack, so there's no good reason to ask
the X server to do restacking that has no effect. (Restackings that have no
effect on either X windows or Wayland windows were generally optimized out in
the synchronization code, but in other cases like moving an X window that is
only beneath Wayland windows to the top of the stack we would make such
requests.)

Removing such requests:
 - Is a small efficiency win in itself
 - Allows us to immediately go ahead and apply Wayland changes to the verified stack
 - Prevents queued Wayland changes piling up waiting for an X event that will never
   be received, since the X server will not send confirmation of no-op restacks.

Since such operations may still have an effect on the relative stacking of X
and Wayland windows, we need to continue applying them to the local stack.

https://bugzilla.gnome.org/show_bug.cgi?id=736559