title: Mapbox GL Cluster location markers show/hide zoom issue
body: <p>I've followed the cluster example. I have three layers. clusters, unclustered-point and cluster-count. When I zoom in the cluster circles and count text are removed fine BUT NOT all the unclustered-point markers show. They show/hide depending on the zoom level. I want ALL unclustered-point markers to show when the cluster circle is removed. You can see what I mean here in this mp4 example. <a href="http://accessibelapp.sensibel.org/map-cluster-issue.mp4" rel="nofollow noreferrer">http://accessibelapp.sensibel.org/map-cluster-issue.mp4</a> and here is my code (sorry about the bad formatting.</p>
  
  <pre><code>map.addLayer({
      id: "clusters",
      type: "circle",
      source: "posNegPoints",
      filter: ["has", "point_count"],
          paint: {
              "circle-color": [
                  "step",
                  ["get", "point_count"],
                  "#51bbd6",
                  100,
                  #f1f075",
                  750,
                  "#f28cb1"
                  ],
                  "circle-radius": [
                      "step",
                      ["get", "point_count"],
                      20,
                      100,
                      30,
                      750,
                      40
                   ],
                   "circle-stroke-width": 2,
                   "circle-stroke-color": "#ffffff"
              }
          });
  
  map.addLayer({
                      id: "cluster-count",
                      type: "symbol",
                      source: "posNegPoints",
                      filter: ["has", "point_count"],
                      layout: {
                          "text-field": "{point_count_abbreviated}",
                          "text-font": ["DIN Offc Pro Medium", "Arial Unicode MS Bold"],
                          "text-size": 12
                      }
                  });
  map.loadImage('http://localhost/sensibel-app/img/good.png', function(error, image) {
                      if (error) throw error;
                      map.addImage('good-icon', image);
                  });
  
                  map.loadImage('http://localhost/sensibel-app/img/bad.png', function(error, image) {
                      if (error) throw error;
                      map.addImage('bad-icon', image);
                  });
  
  map.addLayer({
                      id: "unclustered-point",
                      type: "symbol",
                      source: "posNegPoints",
                      "layout": {
                          "icon-image": "{PosNeg}-icon" // this could be the issue?
                      }
                  });
  </code></pre>
  
  <p>I think it might have to do with my custom icon-image?</p>