destroy() and other clean-ups

[my2iu]

Look at how to clean-up resources properly. Perhaps make glTF files into Texture objects so that they can reuse the Texture garbage collection stuff too?

[my2iu]

It looks like destroy() isn't too important since it only cleans-up stuff that could be cleaned up through normal garbage collection. I did implement a dispose() for cleaning up GL state. I should probably implement multiple ways for it to be called, like Sprite does though.