Line-Line Intersection (2d)

Calculate intersection point of two 2d lines specified with 2 points each

Calculate intersection point of two 2d lines specified with 2 points each
(X1, Y1; X2, Y2; X3, Y3; X4, Y4), while 1&2 and 3&4 specify a line.
Gives back NaN or Inf/-Inf if lines are parallel (= when denominator = 0)
see http://en.wikipedia.org/wiki/Line-line_intersection