Proof-of-concept: Instanced rendering

[codyebberson]

See discussion #78

This is a quick-and-dirty proof-of-concept of using instanced rendering to draw sprites.

Before: https://killedbyapixel.github.io/LittleJS/examples/stress/

After: https://codyebberson.github.io/LittleJS/examples/stress/

Pros: Faster, less data sent to GPU per frame

Cons: Not compatible with glDrawPoints or glRenderPostProcess - to use both instanced rendering and preserve backwards compatibility would require additional shaders.

[KilledByAPixel]

Fantastic work, Cody! I am checking this out now. Im sure I can get the post process working again, we can cut gldraw points as a feature if necessary, i had only added it because it fit so naturally with the old method.

I will fix up the post process to work.

[KilledByAPixel]

It seems to work great already so I'm going to pull this all in then go through and make any small changes i want and fix the post processing and cut the glDrawPoints.