code: function renderCallback(){
  
    while(TO_RENDER.length){
      TO_RENDER.shift().notify();
    }
  
    TO_RENDER.added = {};
  
    while(ENV_QUEUE.length){
      let env = ENV_QUEUE.shift();
      for(let key in env.revalidateQueue){
        env.revalidateQueue[key].revalidate();
      }
      reslot(env);
    }
    ENV_QUEUE.added = {};
  }
docs: Called on animation frame. TO_RENDER is a list of lazy-values to notify. When notified, they mark themselves as dirty. Then, call revalidate on all dirty expressions for each environment we need to re-render. Use `while(queue.length)` to accomodate synchronous renders where the render queue callbacks may trigger nested calls of `renderCallback`.