fix serious performance problem of Animate.

this is a serious bug, it cause massive cpu consume.
if we have 1000 animate in active scene, this pr can raise 5% performance globally, raise about 70% performance locally of Animate!!!!!!!!!!!! .
because it have massive calls cocos2d::Vector constructor and de-constructor. 
Vector's constructor will also call "addRefForAllObjects"(for loop)!!!!!


This problem is very, very serious,  and still has many similar errors in cocos2d-x.

we must use "auto&" sometimes, especially in update per frame. 
use insert "at" of operator[].
I agree with that.

This is true for map but not true for vector. Right?  yes.
 but we'd better use "at" everywhere, reduce the "technical difference",, such use "at" with map, use "[]" with vector .