CROFSU ( undrly, dstnce, offsur, ifail )
========================================

  Create an offset surface.

Receives:
  KI_tag_surface           *undrly         --- underlying surface
  double                   *dstnce         --- offset distance

Returns:
  KI_tag_surface           *offsur         --- offset surface
  KI_cod_error             *ifail          --- failure code

Specific errors:
  KI_su_self_intersect      surface would be self-intersecting
  KI_cant_offset            underlying surface cannot be offset

Description:
  An offset surface is returned - this is defined to be the smooth, continuous
  (G1) surface obtained from the underlying surface by adding to each point on
  it the unit normal vector scaled by the offset distance. If the offset
  distance is zero then the underlying surface is returned, otherwise a new
  surface is created.

  Whenever possible the surface created is not explicitly of type "Offset
  surface", but a simpler analytic type. In these cases CROFSU will not allow the
  caller to specify an offset distance such that the resulting surface would be
  inside out.  For example a sphere cannot be offset inwards by more than its
  radius.

  For surfaces of type TYSUFG, the surface will only be created if the underlying
  surface is not degenerate.

  For surfaces of type TYSUPA, TYSUSE, TYSUSU, the surface created will be
  checked for self intersections caused by degeneracies on the underlying
  surface.  If such a self intersection is detected then an attempt will be made
  to trim the parameter range of the created surface to ensure that it is not
  self intersecting.  If this fails then the surface will not be created.

  Note that if there are any degeneracies on the underlying surface, then the
  bound classification returned by ENSUPA for the underlying surface and for the
  created surface may be different.

  No further checks for self intersection are made by CROFSU. However, the created
  offset surface may be checked for self intersection by calling CHCKEN with the
  appropriate option set (see CHCKEN and SEINTP).

  If the SLIPCO interface parameter is zero, surfaces of type TYSUPA (B-surface)
  will be checked to ensure that they satisfy the composite geometry conditions
  which would be performed by CHCKEN if they were attached to a face. If this
  check fails then the offset surface will not be created.

  The ifail KI_cant_offset is returned if the underlying surface is unsuitable
  for offsetting by any distance. The ifail KI_su_self_intersect is returned if
  the underlying surface is suitable but the requested offset is too large.

  If 'undrly' is not an orphan, then the offset surface will be created as
  construction geometry in the owning part of 'undrly'.

  Surfaces are not allowed to have more than one offset (of type TYSUOF).
  Consequently, 'offsur' may be created as an offset of a copy of 'undrly'.

  Note that if CROFSU creates a surface of type TYSUOF, the underlying surface
  of 'offsur' will be a dependent of it, with all the limitations that this
  implies. In particular, if the offset surface is deleted, then the underlying
  surface may also be deleted. (See DELENT.)