portalsLayers should be an IITC internal use only.

Currently, the best way to loop through the portals and find out which are on the map is
```javascript
    // loop through level of portals, only draw if the portal is shown on map
    for (var guid in window.portals) {
      var portal = window.portals[guid];
      if (portal._map) {
        //on map - draw resonators
      }
    }
```