only ask for a new item when it becomes needed, not while it is needed

Without this change it asks constantly whilst there is something on the marquee, otherwise it stops, which is inconsistent

src/marquee.js

@@ -30,6 +30,7 @@ export class Marquee {
     });
 
     this._waitingForItem = true;
+    this._askedForItem = true;
     this._nextItemWouldBeTouching = startOnScreen;
     this._rate = rate;
     this._lastEffectiveRate = rate;
@@ -142,6 +143,7 @@ export class Marquee {
         throw new Error('Item already exists.');
       }
       this._waitingForItem = false;
+      this._askedForItem = false;
       this._pendingItem = new Item($el, this._direction, metadata, () =>
         this._tickOnRaf()
       );
@@ -393,7 +395,8 @@ export class Marquee {
 
       this._updateWindowInverseSize();
 
-      if (requireNewItem) {
+      if (requireNewItem && !this._askedForItem) {
+        this._askedForItem = true;
         let nextItem;
         this._onItemRequired.some((cb) => {
           return deferException(() => {