Fixed #3249 (#3250)

Only draw non-visible items once when they are loaded, instead of continuously every frame

lib/timeline/component/Group.js

@@ -257,7 +257,7 @@ Group.prototype.redraw = function(range, margin, forceRestack) {
       var me = this;
       var limitSize = false;
       util.forEach(this.items, function (item) {
-        if (!item.displayed) {
+        if (!item.dom) { // If this item has never been displayed then the dom property will not be defined, this means we need to measure the size
           item.redraw();
           me.visibleItems.push(item);
         }