Typo in WebGLRenderGroup?

`PIXI.WebGLRenderer.initStrip` should be `PIXI.WebGLRenderGroup.initStrip`, unless I'm missing something about how `WebGLRenderGroup` is implemented (which is not unlikely).

By the way, what's the purpose of `WebGLRenderGroup`, in brief terms? I don't remember seeing this in previous versions, and it's a pretty substantial addition!