Add FXAAPass class

[neilrackett]

The Three.js FXAA implementation is fantastic, but the WebGL pass has always felt unnecessarily cumbersome and more complicated to use than it needs to be.

This pull request simplifies the creation and use of an FXAA pass by creating a new FXAAPass class that works in the same way as SMAAPass: rather than having to pass FXAAShader as a parameter of the ShaderPass class and manually manage the size of the pass, you simply create a single class instance which is automatically updated by the EffectComposer calling its setSize method.