Since the tangent attributes are now recomputed inside the loop over uv
layers, looping directly over this mesh data is dangerous, as computing
the tangents attributes might trigger some re-allocations, leading to
invalid memory accesses when accessing the next uv layer.
Solved by caching uv layers' names in a list beforehand, the uvlayers
themselves are not even needed for tangent data handling.
Regression from e0a9e98cbd.
Bastien Montagne
a3491762b5