Each time I think I have reach the limits of the crappyness of this "format", it manages to amaze
me again! This time, looks like SDK expects always the same values in the 'templates' (Definitions),
else it declares properties as invalid! One would have to explain me the interest/need to write
those default values in the file, up to this point...

Anyway, went over all defaults I could gather from 'official' FBX files, and replaced the few
ones I had change (because it was better suited to Blender's data). Hopefully it'll be OK now.

Thanks to FslNopper (Norbert Nopper), which helped me troubleshooting this stuff!

There is a good thing here though - it made me find how textures are linked to UVMaps
(by mere names, 'UVSet' property of texture nodes).