Materials are broken when merging a mesh with another that has negative scale

Licencee states that bisMirrored in FMeshMergeHelpers::TransformRawMeshVertexData seems to be involved.

Does not occur in 4.17

Re-confirmed for 4.22 regression in 4.23 MAIN @ CL 6074663

Steps to Reproduce

1. Import or Create a mesh with multiple material channels (attached)
2. Place Mesh
3. Place another mesh, mirror it with Scale(-1,1,1), Scale(1,-1,1), Scale(1,1,-1), or Scale(-1,-1,-1)
4. Merge both actors with default settings
5. Observe strangeness with materials in merged mesh

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Fix Commit6252335
Main Commit4062611
CreatedMar 27, 2018
ResolvedMay 2, 2019
UpdatedOct 15, 2019
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