[osg-submissions] try doubles for math in LineSegment::intersect() methods

[Andy Skinner]

This is to address what I've been asking about on the users list.  We've
got cases where the float math just isn't enough to do the intersections
properly.  I had to convert everything to doubles for it to work.

One optimization is that Vec3d versions of _s, _e, and _s-_e could be
stored in the LineSegment and calculated when needed in an intersection.
The intersect() method is const, so I didn't do it here.

I am a little less sure of the intersect method that takes 3 Vec3s and a
float reference.  I saw it did a-b one place and b-a another, and I used
a Vec3d for this and used its operator- for the second occurrence.  I'm
not sure how much of this is worth it.

If those who do a lot of these intersections could time this and tell me
how it does, I'd appreciate it.

andy

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