function incrementLoaded() {
      countLoaded++;
      if (countLoaded == countTotal) {
         _PRELOADING = false;
      }
      if (countLoaded % 10 === 0) {
         gamejs.logging.debug('gamejs.image: preloaded  ' + countLoaded + ' of ' + countTotal);
      }
   }
preload the given img URIs
@returns {Function} which returns 0-1 for preload progress
@ignore