code: function getSpikeShape(coord) {
    var x = coord.x + 20, y = coord.y + 20;
    var spike_radius = 14;
    // almost = spike_radius * tan(pi/8) for the vertices of a regular octagon.
    var point_offset = 5.8;
    return [
      {x: x - spike_radius, y: y - point_offset},
      {x: x - spike_radius, y: y + point_offset},
      {x: x - point_offset, y: y + spike_radius},
      {x: x + point_offset, y: y + spike_radius},
      {x: x + spike_radius, y: y + point_offset},
      {x: x + spike_radius, y: y - point_offset},
      {x: x + point_offset, y: y - spike_radius},
      {x: x - point_offset, y: y - spike_radius}
    ];
  }
docs: Given an x and y value, return a polygon (octagon) that approximates a spike at the tile given by that x, y location. Points in CW order.