private static void intersectY(MutableCoordinateSequence out, double ax, double ay, double bx, double by, double y) {
    double t = (y - ay) / (by - ay);
    out.addPoint(ax + (bx - ax) * t, y);
  }
Adds a new point to {@code out} where the line segment from (ax,ay) to (bx,by) crosses a horizontal line at y=y.
planetiler-core/src/main/java/com/onthegomap/planetiler/render/TiledGeometry.java