Tesselation optimization #1: selective tesselation

  - there is no need to tesselate flat terrain so it is a
    waste of time to do so.
  - implemented a crudely working system to vary the tess levels
    issued by the TESC shader for patches with less height variation.
    -> drops GPU demand to (36/56) <20% (from 30%+).
    -> uses height diffs from 3 patch corner verts to set variation
       but this produces artifacts along edges that are in varying
       terrain but coincidentally match heights.
    -> ideally need a system that averages between patch demands, but
       how to share patch info???

  * this system also eliminates all cracking and seam gaps *