Added support for assets manager for textures, texture atlases, and bitmap fonts

Particle effects would require a bigger refactor, probably going to hold off on that for now
You can also manually use the asset manager for everything, as well as pixmaps, particle effects, music, sound, and skins

I could also take it out of g2d, and players can use it manually, ie:  (texture (assets/texture-asset "tilemap.png")) works just as well