Geometric functions concerning lines.
Given an infinite line which intersects
return the point on that line that is closest to
Given an infinite line which intersects P1 and P2, let P4 be the point on the line that is closest to P3.
Return an indication of where on the line P4 is relative to P1 and P2.
if P4 == P1 return 0 if P4 == P2 return 1 if P4 is halfway between P1 and P2 then return 0.5