title: Making particles "twinkle" with a custom shader
body: <p>I'm making a night sky and I'd like the stars in my ParticleSystem to twinkle.</p>
  
  <p>I've heard that I can use a noise texture for randomness, but I'm not sure where to go from there.  And I'm not sure how to make the twinkle happen smoothly, ie. get brighter gradually and then dimmer.</p>
  
  <p>Right now I am just giving each vertex a random <code>rand</code> attribute, and then changing a uniform <code>twinkleRand</code> every 200ms or so.  If the randoms match, the point becomes larger:</p>
  
  <pre><code>if (rand == twinkleRand) {
     gl_PointSize = size * abs(sin(js_time) + rand/100.);
  }
  else {
     gl_PointSize = size;
  }
  </code></pre>
  
  <p>It's sort of twinkly but not quite a convincing effect.</p>