Fix LOD generation for meshes with tangents & mirrored UVs

When UVs are mirrored in a mesh, collapsing vertices across the
mirroring seam can significantly reduce quality in a way that is not
apparent to the simplifier. Even if simplifier was given access to UV
data, the coordinates would need to be weighted very highly to prevent
these collapses, which would penalize overall quality of reasonable
models.

Normally, well behaved models with mirrored UVs have tangent data that
is correctly mirrored, which results in duplicate vertices along the
seam. The simplifier automatically recognizes that seam and preserves
its structure; typically models have few edge loops where UV winding is
flipped so this does not affect simplification quality much.

However, pre-processing for LOD data welded vertices when UVs and
normals were close, which welds these seams and breaks simplification,
creating triangles with distorted UVs.

We now take tangent frame sign into account when the input model has
tangent data, and only weld vertices when the sign is the same.

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