private static boolean slanted(int x1, int y1, int x2, int y2) {
      return x1 != x2 && y1 != y2;
    }
Returns {@link #LEFT} or {@link #RIGHT} bitwise OR'd with {@link #TOP} or {@link #BOTTOM} to indicate which side
of the rectangle from {@code (x=min, y=min)} to {@code (x=max, y=max)} the {@code (x,y)} point falls in.
planetiler-core/src/main/java/com/onthegomap/planetiler/VectorTile.java