FBX IO: Export normals matching the mesh's normals_domain

The appropriate normals array and FBX layer mapping are now used
according to the Mesh.normals_domain property.

This results in slightly faster exports with smaller file size when
meshes are either fully smooth shaded or fully flat shaded because the
FACE and POINT domains almost always have fewer normals than the CORNER
domain and never have more (a fully smooth shaded mesh where each face
has its own vertices would be POINT domain with as many vertices as
corners).

Additionally, getting the normals from the new Mesh.corner_normals
property runs in about 60% of the time of getting the normals through
MeshLoop.normal.

For me with a subdivided cube with 185856 corners:
MeshLoop.normal: ~6.7ms
Mesh.corner_normals: ~4.0ms

Import is unaffected, because only custom split normals can be set.

@@ -1176,3 +1194,3 @@
             # Since we don't care about how the normals are sorted, only that they're unique, we can use the fast unique
             # helper function.
-            t_ln, t_lnidx = fast_first_axis_unique(t_ln.reshape(-1, 3), return_inverse=True)
+            t_normal, t_lnidx = fast_first_axis_unique(t_normal.reshape(-1, 3), return_inverse=True)