function pinch_center_x(e) {

        var touch_a = e.touches[0];
        var touch_b = e.touches[1];

        return (touch_a.clientX + touch_b.clientX) / 2;

    }
Given a touch event, calculates the center between the first two
touches in pixels, returning the X coordinate of this center.

@param {TouchEvent} e The touch event to use when performing center 
                      calculation.
@return {Number} The X-coordinate of the center of the first two touches.