widget: Reset alloc_needed_on_child before allocating children

widget: Reset alloc_needed_on_child before allocating children

Reset alloc_needed_on_child *before* allocating the children. This is
because some child's size_allocate() may call queue_allocate(), which
will bubble up alloc_needed_on_child. An example of this happening is
with GtkScrollable implementations, which are supposed to configure
their adjustments in size_allocate(), which will cause GtkScrollbar's
GtkRange to notice and queue_allocate() on itself.

If we reset alloc_needed_on_child after this happens, then our children
will have a lingering alloc_needed_on_child and will not receive an
allocation.

This commit fixes widgets occasionally losing an allocation when this
scenario happens.