code: function loadAll (urls, callback, sendCallbackWhenAlreadyLoaded) {
          // list all urls which are not yet loaded
          var urlsLeft = [];
          urls.forEach(function (url) {
              if (!isLoaded(url)) {
                  urlsLeft.push(url);
              }
          });
  
          if (urlsLeft.length) {
              // there are unloaded images
              var countLeft = urlsLeft.length;
              urlsLeft.forEach(function (url) {
                  load(url, function () {
                      countLeft--;
                      if (countLeft == 0) {
                          // done!
                          callback();
                      }
                  }, sendCallbackWhenAlreadyLoaded);
              });
          }
          else {
              // we are already done!
              if (sendCallbackWhenAlreadyLoaded) {
                  callback();
              }
          }
      }
docs: Load a set of images, and send a callback as soon as all images are
  loaded
  @param {String[]} urls
  @param {function } callback
  @param {boolean} sendCallbackWhenAlreadyLoaded