This separates the UV reverse sampling and the barycentric mixing of
the mesh attribute into separate multi-functions. This separates
concerns and allows for future de-duplication of the UV sampling
function if that is implemented as an optimization pass. That would
be helpful since it's the much more expensive operation.
This was simplified by returning the triangle index in the reverse
UV sampler rather than a pointer to the triangle, which required
passing a span of triangles separately in a few places.
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