The vertex normals and tangent space attributes are not currently exposed by
the XML API, in the Mesh node. This notably prevents the XML API user
from applying normal maps to meshes in tangent space.
To overcome this situation, this commit adds three XML attributes to the
Mesh node: 'N' (vertex normals), 'tangent' and 'tangent_sign'.
Nota: at the moment, these attributes are only available for non-subdivided
meshes.
