class TXTRU: public TShape

 XTRU is an poly-extrusion with fixed outline shape in x-y,
 a sequence of z extents (segments) and two end faces perpendicular
 to the z axis.  The x-y outline is defined by an ordered list of
 points; the overall scale of the outline scales linearly between
 z points and the center can have an x-y offset specified
 at each segment end.

 A TXTRU has the following parameters:

     - name       name of the shape
     - title      shape's title
     - material  (see TMaterial)
     - nxy        number of x-y vertex points constituting the outline --
                  this number should be at least 3
     - nz         number of planes perpendicular to the z axis where
                  the scaling dimension of the section is given --
                  this number should be at least 2
     - Xvtx       array [nxy] of X coordinates of vertices
     - Yvtx       array [nxy] of Y coordinates of vertices
     - z          array [nz] of z plane positions
     - scale      array [nz] of scale factors
     - x0         array [nz] of x offsets
     - y0         array [nz] of y offsets

 Author:  R. Hatcher 2000.04.21

 All XTRU shapes are correctly rendered in wire mode but can encounter
 difficulty when rendered as a solid with hidden surfaces.  These
 exceptions occur if the outline shape is not a convex polygon.
 Both the X3D and OpenGL renderers expect polygons to be convex.
 The OpenGL spec specifies that points defining a polygon using the
 GL_POLYGON primitive may be rendered as the convex hull of that set.

 Solid rendering under X3D can also give unexpected artifacts if
 the combination of x-y-z offsets and scales for the segments are
 chosen in such a manner that they represent a concave shape when
 sliced along a plane parallel to the z axis.

 Choosing sets of point that represent a malformed polygon is
 not supported, but testing for such a condition is not implemented
 and thus it is left to the user to avoid this mistake.