function boundingBox(contours) {
  let minX = 3000, maxX = -3000, minY = 3000, maxY = -3000;
  contours.forEach(contour => contour.forEach(pt => {
    if (pt.x < minX) minX = pt.x;
    if (pt.x > maxX) maxX = pt.x;
    if (pt.y < minY) minY = pt.y;
    if (pt.y > maxY) maxY = pt.y;
  }));
  return { minX, maxX, minY, maxY };
}
Get the bounding box of a glyph contour
@param { GlyphData } contours