Tess optimization #1: improved selective tesselation

  - selection of tess level now based on a sample 'tess_density' value
    for each corner vert of the patch.
  - the 'tess_density' sample comes from the 'g' value of the terrain map.
  - this value is assigned a tesselation level based on the thresholds
    given in the 'geom_lod' of the planet.
  * all artifacts eliminated
  * minor drop in GPU demand to (34/54) 18.3 from (36/56) 20.