class HTriMesh: public TriMesh

 Hierarhical mesh

 Consists of:
 a) high-level hierarhical subdivision
    all hierarchy represented by a single TringTvor;
    nodes point into this tvor to identify triangles;
    ?? depth first or breadth first ??
    breadth first keeps triangles from a given level together
    - good for opcode, to construct full tree at some level;
      is this good? is it needed?
    - also good for rendering, as some level will be selected at a given
      distance (in space approach mode);
    depth first keeps one brach together
    - good for recursive queries / descends?
 b) low-level triangle soup attached to a leaf-node
    for now, keep all of them in own TringTvor

 In principle could have several a-like layers.
 Or simplify the thing by always having a single a-layer -- but this then
 leads to duplication of vertices.

 Hmmh ... aren't vertices always shared?
 Yes ... but at some point I'd like to move leaf-vertices into separate
 meshes, managed by separate tringulas.

 So? At this intermediate level ... what do I do?
 Duplicate a-level vertices when creating b-mesh -- to keep Tringula ok.
 Then, it will also become important if one sees Flyers -- checking whole
 bounding volume of the triangle. Hmm, all Extendios ... but they don't go
 that hight so one could just hack highest point.