Added AlternativeNormalPass option for StandardEffects

[giulioz]

As shown in Issue #18, when you combine instancing and SSAO the rendering breaks, since it uses internally scene.overrideMaterial. It's not possible to use a single material for instanced and non instanced objects, since the program attributes are different.

I've noticed that using this approach it's pretty difficult to make a fix, so I've found an hack: you can render a Normal Map, even with instancing, only if you use separate MeshNormalMaterials, traversing the scene tree and temporarily exchanging the materials.

I acknowledge that this can be a corner case or an hack, but it may be useful for someone…

[drcmda]

Could you put this one into react-postprocessing instead? I'll remove standardeffects when it's out

[giulioz]

I'll open an issue, since it's more complicated that that unfortunately…