Lightmap UV set generator

The assetimport module should have a helper class, similarly to e.g. QSSGMeshBVHBuilder, that takes a QSSGMeshUtilities::Mesh and generates, using the xatlas third-party library, an additional UV channel (UV2 or whatever we decide to call it) for each sub-mesh in the mesh.

It should be investigated what is the best apporoach for feeding the resulting UV data back into the input Mesh. This also involves introducing support for the additional UV set.

It should be evaluated also what to do for custom geometry: for example the mentioned QSSGMeshBVHBuilder features an alternative code path (a constructor that takes the vertex/index data and associated metadata as plain QByteArray and other arguments). It it more complicated in his case because our output is some data that should be merged back into the input vertex buffer data somehow. It could be that this is best postponed for the time being, and do not enable and promote custom geometry with lightmaps for now.