AssetManager, refactoring and cleanup.
AssetManager, refactoring and cleanup.

* Stack -> Array.
* Better use of iterators.
* Use ObjectIntMap#getAndIncrement.

stinsonga/libgdx,tommyettinger/libgdx,tommyettinger/libgdx,bladecoder/libgdx,NathanSweet/libgdx,stinsonga/libgdx,stinsonga/libgdx,NathanSweet/libgdx,stinsonga/libgdx,cypherdare/libgdx,libgdx/libgdx,tommyettinger/libgdx,bladecoder/libgdx,cypherdare/libgdx,stinsonga/libgdx,bladecoder/libgdx,libgdx/libgdx,tommyettinger/libgdx,NathanSweet/libgdx,Zomby2D/libgdx,bladecoder/libgdx,libgdx/libgdx,tommyettinger/libgdx,bladecoder/libgdx,libgdx/libgdx,Zomby2D/libgdx,Zomby2D/libgdx,cypherdare/libgdx,libgdx/libgdx,cypherdare/libgdx,NathanSweet/libgdx,NathanSweet/libgdx,cypherdare/libgdx,Zomby2D/libgdx,Zomby2D/libgdx