atgc-core-tween: implement our own tween engine

[jbilcke]

creating thousand of new tweens is a costly operation because it involves a lot of prototype instancing, playing with lambdas, and memory writing, during gameplay the framerate drops quickly :(

it would more efficient to have a pre-allocated array of tweens, pre-initialized with same functions, and recycle them. also, maybe we should compare the function to just reusing existing one (ie. if we have 100000 tweens, to not create 100000 lambdas but reuse the same)

actually I need to check the inside of tween.js, either we fork tween.js to add ra .recycle() function, or we create our own, if the tween.js architecture doesn't allow this kind of modification.