The current function generates a large number of errors about invalid
UV coordinates when run.  It sets the u and v coordinates to the X and
Y coordinates of the hit_vpriv vector, which is set in
rt_extrude_shot, but vpriv is not always related directly to the hit
point of the ray: for example, if the ray hits a line segment, vpriv
is set to tmp, which is the difference between the start and end
vertices of the line segment.  This can easily result in out-of-range
u and v coordinates; for coordinates in the range, the resultant
mapping is not directly related to the hit point on the geometry of
the object, and it definitely doesn't generate a smooth/continuous
mapping of the image over the whole object.

Unfortunately, within the (relatively small) amount of time remaining
for GCI, I haven't been able to locate any examples of pre-existing uv
mappings for extruded objects, or develop a mapping that I think is
satisfactory (I would like it to be both smooth and continuous, and
not tile the image over the surface, which results in sharp edges
(possibly on the edges of the extruded object) between copies of the
image).  I therefore haven't been able to include a patch that fixes
the issue/renderings that show the fix; however, after I finish my
other projects from during GCI, I'll try sending an email to the
brlcad-devel mailing list and seeing if anyone has any ideas for a
mapping.  If I can think of/find a workable approach for the uv
mapping, I'll be happy to implement it.