Function getScale

docs/tests/v2/es6/js/sanddance.js:52131–52133

* calculate the scale factor between two pointersets * no scale is 1, and goes down to 0 when pinched together, and bigger when pinched out * @param {Array} start array of pointers * @param {Array} end array of pointers * @return {Number} scale

(start, end)

  function getScale(start, end) {
    return getDistance(end[0], end[1], PROPS_CLIENT_XY) / getDistance(start[0], start[1], PROPS_CLIENT_XY);
  }

  var MOUSE_INPUT_MAP = {
    mousedown: INPUT_START,