Get the center point coordinate of a group of pointers.
@param {Object} pointers - The target pointers.
@returns {Object} The center point coordinate.
function getPointersCenter(pointers) {
  var pageX = 0;
  var pageY = 0;
  var count = 0;
  forEach(pointers, function (_ref3) {
    var startX = _ref3.startX,
        startY = _ref3.startY;
    pageX += startX;
    pageY += startY;
    count += 1;
  });
  pageX /= count;
  pageY /= count;
  return {
    pageX: pageX,
    pageY: pageY
  };
}