The material has the "isDoubleSided" bool parameter. Because some models have double-sided texture settings, the new bool parameter is added to determine whether glTF2 'doubleSided' is enabled. The default value of this parameter is false.
At the end of the various material/mesh transformations we do, we were still using a ridiculously simplistic method of mapping RawMaterial to glTF MaterialData.
This switches to using FBX's GetUniqueID(), which should be the law of the land in general. Other model entities may need further investigation as well.
We don't want to use the logical texture name from the FBX to distinguish separate textures. It seems entirely valid for distinct textures to have the same name. It's not completely clear what's best to use instead, but adding fileLocation seems reasonable. It can be blank, for textures that don't exist on the filesystem –– but that's a pretty weird edge case, and even then we have both 'usage' and 'name' to distinguish.
Did not mean to commit/push the current state of master. But rather than
mess up source control history with a force push, I'll just try to hurry
to a stable point.