Triindices for vertex identification

Triindices can directly be translated from one division into another, as
opposed to biindices (i and j). Also, it is not sufficient to check for
the proximity of the three corner vertices of a subpatch, in order to
determine whether it is in range. That is, if we require that the
visiblity is really determined by the lower LOD.
If performance demands, the check may be reduced to only the three
corner vertices as an approximation (which is reasonable if the surface
is somewhat smooth, i.e. the hight of peaks within a patch is limited by
the extent of the patch).