In that code setion it is already checked that the line segments (a,b) and (c,d)
are parallel. At this point it is also known that the line segment (a,c) is also
parallel to (a,b). In that case it is possible to express c as
c(t) = a + t * (b-a) (analog for the point d). Since the evaluation of all three
coordinate equations (x,y,z) have to lead to the same solution for the parameter
t it is sufficient to compute t only once.

Before the computation of the parameter was only performed for the x coordinate
equation. If the line segment are perpendicular to the x-axis this leads to
problems. The changes at hand treat this case.

The condition in the comment above the changed lines was not correct checked.

many are superfluous, some could be turned into
fwd decls, some wrongly documented.

The special case where the line segments (a,b) and (c,d) are parallel but are not
located at the same straight line was not handled properly. This led to a
singular matrix and to a not a number solution. This commit fixes the
handling of the special case.

If two line segments are parallel, but do not intersect each other is handled now correct.

In case the normal is already parallel to z axis the previous algorithm failed.