Returns the components of the surface normal specific UV coordinates.
Usage
normal(surface_node, prim_num, u, v, index)
u and v are unit values, defined in the [0,1] interval.
If the primitive is a mesh, u and v are defined in terms its number
of rows and columns.
index specifies which component to return, where 0 means X, 1 means Y,
and 2 means Z.