Fix spine::Skeleton not updating on re-entry.

Could be done in a tighter manner by scheduling updates in
spine::SkeletonAnimation instead and listening for animation events.
However, this may need a more robust interface on scheduleUpdate and
unscheduleUpdate to work correctly.
Merge pull request #5461 from Wilhansen/spine_skelFix

closed #4156: spine::Skeleton would not be updated after re-adding to scene.