Save generated tangents on export

[wheeland]

Task-Id: KUE-617

[wheeland]

As of now, I kept this in GLTF2Importer. Doesn't feel like the best place to put it though. Moving everything to GLTF2Exporter doesn't make sense of now, as this one is only created once during export, and we want to see the tangents in action within APE, as materials might depend on them, right?

I guess one way to make it cleaner would be to move the tangent computation itself into GLTF2Context for example, retain some information on what has changed, including a new QByteArray for the newly generated buffer(s), and then in GLTF2Exporter use all that saved info to modify the JSON tree. This would require some slightly awkward duplication of information though, which is not needed as of now.

Trying to figure out what breaks the Draco export now. Does the DracoExportPass expect all data to be stored inside a single buffer file?

[mkrus]

could use some unit tests