Remove disney roughness reparameterization

[bhouston]

This PR is built upon the cosine cubemap LOD PR: #7414

I have removed the restriction of the Physical materials roughness parameterization to just [0.5 to 1], rather allowing it to vary between [0.04, 1] like the roughness parameterization in UE4. (The lower limit of 0.04 is required to avoid some divide by zeros.) This allows one to make perfectly shiny mirrors and other perfectly polished metals.

[mrdoob]

@WestLangley thoughts?

[WestLangley]

We need to address this issue after the dev branch is working again.

It seems unlikely to me that Disney would get this wrong, but we will see.

In any event, in the API, roughness ranges from zero to 1. The objective is to make roughness "perceptually linear" over that range. So, the [ 0, 1 ] input is remapped in the shader so we get the perceptually linear result we want.

[bhouston]

I do not change the linearity, I just extended the range. If something isn't compiling I can look into it.

[WestLangley]

I think the commit that is the topic of this PR is bhouston@e7db59e

That change is fine with me.

[mrdoob]

Thanks guys!

[mrdoob]

I've been playing with the new material and I was starting to notice that the sharper material I was able to achieve was metal. This change brings many more possibilities indeed!

[mrdoob]

By the way... Seems like when I use metalnessMap, metalness has no effect. But when using rougnessMap, roughness still has effect. Is this the exected behaviour?

[WestLangley]

We can change the behavior so metalnessMap modulates metalness. That would be consistent with how the other maps are handled.

Or we can change how roughness is handled.

It is up to you.

[mrdoob]

I think I vote for metalnessMap modulating metalness. Just like map modulates color.

[WestLangley]

OK. I'll take care of it.

[mrdoob]

Thanks!