Implement texture atlassing

[davedx]

We should support texture atlassing, where we draw sprites from an atlas by modifying the U,V buffer for each different sprite.

TODO:

• [x] Blit loaded images to atlas automatically
• [ ] Manage creating new atlasses on demand, when last one is full
• [x] Generate correct U/V's for each buffer and add ref to texture on the buffer
• [x] Manage binding of atlasses according to which texture is needed
• [ ] Add support for grouping hints

[davedx]

In progress now. Used texSubImage2D to blit dynamically loaded image assets into one texture atlas...

[davedx]

Still not as fast as pixi, but it's working!