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