function scaling(xRatio, yRatio = xRatio) {
  return glyph => glyph.map(contour => contour.map(
    pt => ({ x: pt.x * xRatio, y: pt.y * yRatio, on: pt.on })));
}
Scaling
@param { number } xRatio 
@param { number } yRatio 
@returns { PostFilter }