force rounding to integer the result of map.getZoom() when calculating the map data zoom

it's not something that sould happen, but it's been seen on mobile (#271) - this should eliminate the issues with portal data this causes

code/utils_misc.js

@@ -247,6 +247,9 @@ window.renderLimitReached = function(ratio) {
 window.getPortalDataZoom = function() {
   var z = map.getZoom();
 
+  // on mobile (at least), the map zoom has been non-integer occasionally. fix it.
+  z = Math.floor(z);
+
   // limiting the mazimum zoom level for data retrieval reduces the number of requests at high zoom levels
   // (as all portal data is retrieved at z=17, why retrieve multiple z=18 tiles when fewer z=17 would do?)
   // very effective along with the new cache code