Fix intersect_segments

[kolbyb]

• Added sub support to Point
• Added a few helper functions to Point

  angle: Calculates the angle for this Point from (0, 0). Useful when calculating deltas of Points. cross: Equivalent of calculating a Cross Product on a 3-dimensional (x, y, 0.0) Point. dot: Dot Product between two Points. length: Distance/Length of this Point. normal: A normalized version of this Point.

• Fixed intersect_segments

  Implemented a segment intersection test, instead of relying on the previous line slope intersection test. This will address the intersections not working when axis-aligned.

  The formula used calculates the determinant and the cross products of each Segment's inverse delta.