code: function getCenter(pointers) {
      var pointersLength = pointers.length;
  
      // no need to loop when only one touch
      if (pointersLength === 1) {
          return {
              x: round(pointers[0].clientX),
              y: round(pointers[0].clientY)
          };
      }
  
      var x = 0, y = 0, i = 0;
      while (i < pointersLength) {
          x += pointers[i].clientX;
          y += pointers[i].clientY;
          i++;
      }
  
      return {
          x: round(x / pointersLength),
          y: round(y / pointersLength)
      };
  }
docs: get the center of all the pointers
  @param {Array} pointers
  @return {Object} center contains `x` and `y` properties