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)
	    };
	}
get the center of all the pointers
@param {Array} pointers
@return {Object} center contains `x` and `y` properties