IRID 4D/70g zbuffer bugs

[Steve Dempsey]

When Z buffering is enabled, depthcueing and clipping of line segments is not
always done properly.

When a depthcued line is zbuffered, all of the pixels of the line are
drawn at the same intensity, which appears the be the intensity of the closest
pixel.  It appears that only the first pixel is depthcued, and then all of the
remaining pixels are given whatever color was determined for the first pixel.
This bug is most noticable when long lines are drawn from the hither to yon
plane.  Note that if the line is drawn as a series of short segments then
each segment will be drawn at a different intensity and you might not notice
the problem.

In addition, some depthcued and zbuffered lines are not clipped properly.  
As the hither plane moves behind one endpoint of the line segment, the entire
segment disappears.