[StScrollBar] avoid queueing a relayout during allocation

[StScrollBar] avoid queueing a relayout during allocation

When an StScrollView is allocated, allocating the child would
cause the adjustment values to change, which would result in
the scrollbars queueing a relayout, which isn't allowed during
allocation.

To avoid this, instead of queueing a relayout when the adjustment
changes:

 - When we have a valid allocation already, just go ahead
   and reallocate the children.
 - Otherwise do nothing immediately and wait until we get allocated

Because the 'needs_allocation' flag in ClutterActor isn't exposed,
this requires some slightly ugly code to shadow that state locally.

https://bugzilla.gnome.org/show_bug.cgi?id=611944