Support tangent attributes

[andrewvarga]

Hi! Thanks for this library!
I played with it for a hobby project using in combination with lightmap-baker and I found that if the model has tangent attributes then those are not updated and so cause WebGL drawing errors (because the attribute count changes for position, normal and texture attributes but not for tangents..

I suspect the changes would be around this line..?

xatlas-three/src/UVUnwrapper.ts

Line 151 in 9e8edf7

if(m.vertex.normals) mesh.setAttribute('normal', new this.THREE.BufferAttribute(m.vertex.normals, 3, true));