- base level surface container is now a meshedSurface instead of
  a triSurface. This avoid automatic triangulation of surfaces
  when they are read, and simplifies the internals.

- sampling types:
  * "meshedSurface" (compat: "sampledTriSurfaceMesh")
  * "meshedSurfaceNormal" (compat: "sampledTriSurfaceMeshNormal")