Add unwrap() UV unwrapping function

[donmccurdy]

Resources:

• https://github.com/MozillaReality/xatlas-web
• https://github.com/Microsoft/UVAtlas
• https://github.com/prideout/aobaker
• https://github.com/jpcy/xatlas
• https://github.com/gkjohnson/three-gpu-pathtracer/blob/main/src/utils/UVUnwrapper.js

xatlas seems like the leading choice here, with support for generating a texture atlas or packing one or more existing meshes into a single atlas. The xatlas-web WASM build doesn't seem to expose quite the behavior I'd want (unless I missed something) with what appears to be an assumption that each input mesh has {index, position, normal, uv} and output mesh has {index, position, normal, uv, uv2}. I need to support generic attributes and so need to access the vertex xref to rewrite the mesh after atlasing:

• "... original index of each output vertex - it tells you where to get the data for an output vertex from the input mesh."
• https://github.com/jpcy/xatlas/blob/16ace528acd2cf1f16a7c0dde99c42c486488dbe/source/xatlas/xatlas.h#L58-L65