Fix OBJ mesh importer smoothing handling

[kleonc]

Fixes #71085.

I thought it was a regression from #68034 so fixed the smoothing logic accordingly. Seems like it does fix #71085, with this PR the MRP outputs (after forcing reimport):

4 vertices, 6 indices
Vertices: 
(1, 0, 1)
(-1, 0, 1)
(1, 0, -1)
(-1, 0, -1)
Indices: 
0
1
2
2
1
3

However, #71085 was reported before #68034 was even merged so that was not really the cause... 🤔 Not sure what was the exact cause back then but seems to work now. 🙃

[clayjohn]

Looks good to me. I guess the issue was that all flat shaded sections ended up having different smooth groups so they couldn't be merged (even if they had compatible attributes). The update to the API allows you to assign all flat vertices to the same designated group uint32_t(-1) and still keep them flat while also allowing the SurfaceTool to merge them.

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.0.3.