Compute and fill the normals of the vertex data interleaved array
arr at level given. If array-below is given, when the vertex is shared
with the array in the level below [level-1], copy that directly, to
avoid glitches when switching levels in visible patches.
If terrain-patch is given, normals will take into account vertices
in neighbouring patches to produce seamless normals on the edges,
also normals generated on the edges will be copied into neighbours
at all applicable levels (namely going to higher levels of detail).