Rounding particles with canvas

HTML

<canvas></canvas>

CSS

body { background:white; }

JavaScript

MeteorRain = new function() {
    var SCREEN_WIDTH = window.innerWidth;
    var SCREEN_HEIGHT = window.innerHeight;

    var cursor, canvas, context;

    var particles = [];

    var mouseX = 0;
    var mouseY = 0;

    this.init = function() {
        canvas = document.querySelector("canvas");

        if (canvas && canvas.getContext) {
            context = canvas.getContext('2d');

            document.addEventListener('mousemove', documentMouseMoveHandler, false);
            document.addEventListener('mousedown', documentMouseDownHandler, false);
            window.addEventListener('resize', windowResizeHandler, false);

            createCursor();

            windowResizeHandler();

            setInterval(loop, 1000/100);
        }
    }

    function createCursor(position) {
        var w = 300;
        var h = 300;

        if (!position){
            var pos = {
                x: ( SCREEN_WIDTH - w ) * 0.5 + (Math.random() * w), 
                y: ( SCREEN_HEIGHT - h ) * 0.5 + (Math.random() * h)
            }

            var m = new Cursor();
            m.position.x = pos.x;
            m.position.y = pos.y;

            cursor = m;

            createParticles(m.position);

        } else {
            var m = new Cursor();
            m.position.x = position.x;
            m.position.y = position.y;

            createParticles(m.position);
        }
    }

    function createParticles(pos) {
        for (var i = 0; i < 50; i++) {
            var p = new Particle();
            p.position.x = pos.x;
            p.position.y = pos.y;
            p.shift.x = pos.x;
            p.shift.y = pos.y;

            particles.push(p);
        }
    }

    function documentMouseMoveHandler(event) {
        mouseX = event.clientX;
        mouseY = event.clientY;
    }

    function documentMouseDownHandler(event) {
        createCursor({x: mouseX, y: mouseY});
    }

    function windowResizeHandler() {
        canvas.width = SCREEN_WIDTH;
       ...