Ensure intersection point of lines lies on given segments

[jzbrooks]

The calculation for line intersection was correct, but the intersection point wasn't ensured to lie on both of the line segments. So the calculation was incorrect.

Fixes #60

[jzbrooks]

It turned out this effectively turned the optimization off, which should be done more explicitly if needed.

The line segments probably never actually cross, but we need to compute the intersection of the line, not the line segment.