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