Remove Vector2.tmp() and its variations (.tmp{1,2,3})

This change removes the public static mutable "tmp" objects from `Vector2`.  Mostly they're replaced with *private* static mutable tmp objects elsewhere in the code.  

I could probably reduce the number of scratch objects in the scene2d code, if anyone objects to the number created (I think its 10 new and 3 removed).

There is one free cleanup of a `Vector3` tmp usage while I was in Stage.

This breaks the API since `Vector2.tmp`* were all public.

Some discussion on this topic: http://www.badlogicgames.com/forum/viewtopic.php?f=23&t=7814  Folks are free to change their opinions now that they've got a concrete example.  :)