MeshPhysicalMaterial: Added sheen textures.

[Mugen87]

Related issue: -

Description

To make the sheen code more glTF conform, this PR adds sheenTintMap and sheenRoughnessMap which are the glTF pendants to sheenColorTexture and sheenRoughnessTexture.

[mrdoob]

Before we go too far down this path...

I think it's not too late to rename "Tint" to "Color".
It'd follow material.color which has always acted as a "tint" too.

So the change would be:

attenuationTint > attenuationColor
sheenTint > sheenColor
sheenTintMap > sheenColorMap
specularTint > specularColor
speculatTintMap > specularColorMap

@WestLangley If you don't hate this naming too much I can do the change after merging this 😇

[WestLangley]

@WestLangley If you don't hate this naming too much...

@mrdoob hehe. It's OK with me. :-)

[mrdoob]

Sweet! I'll do the change tomorrow 😀

[mrdoob]

Sweet! I'll do the change tomorrow 😀

Damn, the last two weeks have been intense...

[mrdoob]

Thanks!