Rename _render to _updateLatex in Pixi widget so it isn't called every frame

[hoodmane]

Trying to use this in a pixi.js animation I found it was extremely slow. Problem seems to be that PixiJS._render is called by pixi.js every frame. But we actually only want to call it when the math changes, which is relatively unusual. This patch fixes the performance trouble.

[mattbargar-cainc]

Hi @hoodmane - this looks great and totally makes sense; would you mind just making a comment in this file to say that this new method name corresponds to the _render() method in the createjs version?

[hoodmane]

What about renaming that one too? I think _updateLatex or something in that vein is a more accurate name anyways since this function doesn't need to be called to render the latex, only whenever the latex changes.