function linePolygonIntersect(a1x, a1y, a2x, a2y, points) {
  for (var i = 0, p2 = points[points.length - 1]; i < points.length; i++) {
    var p = points[i];

    if (lineLineIntersect(a1x, a1y, a2x, a2y, p[0], p[1], p2[0], p2[1])) {
      return true;
    }

    p2 = p;
  }
}
Return `true` if the given line (line `a`) and the given polygon
are intersect.
Note that we do not count colinear as intersect here because no
requirement for that. We could do that if required in future.