Improve pre-rendering at the start/end of the document

This is a very old "issue", which has existed since essentially forever, and it affects all of the available scrollModes. However, in the recently added Page-mode it's particularily noticeable since we use a *simulated* scroll direction there.

When deciding what page(s) to pre-render, we only consider the current scroll direction. This works well in most cases, but can break down at the start/end of the document by trying to pre-render a page *outside* of the existing ones. To improve this, we'll thus *force* the scroll direction at the start/end of the document.

*Steps to reproduce:*

 0. Open the viewer, e.g. https://mozilla.github.io/pdf.js/web/viewer.html
 1. Enable vertical scrolling.
 2. Press the <kbd>End</kbd> key.
 3. Open the devtools and, using the DOM Inspector, notice how page 13 is *not* being pre-rendered.

web/base_viewer.js

@@ -1361,10 +1361,12 @@ class BaseViewer {
     return promise;
   }
 
-  /**
-   * @private
-   */
-  get _scrollAhead() {
+  #getScrollAhead(views) {
+    if (views.first.id === 1) {
+      return true;
+    } else if (views.last.id === this.pagesCount) {
+      return false;
+    }
     switch (this._scrollMode) {
       case ScrollMode.PAGE:
         return this._scrollModePageState.scrollDown;
@@ -1376,7 +1378,7 @@ class BaseViewer {
 
   forceRendering(currentlyVisiblePages) {
     const visiblePages = currentlyVisiblePages || this._getVisiblePages();
-    const scrollAhead = this._scrollAhead;
+    const scrollAhead = this.#getScrollAhead(visiblePages);
     const preRenderExtra =
       this._spreadMode !== SpreadMode.NONE &&
       this._scrollMode !== ScrollMode.HORIZONTAL;

web/pdf_thumbnail_viewer.js

@@ -295,12 +295,22 @@ class PDFThumbnailViewer {
     return promise;
   }
 
+  #getScrollAhead(views) {
+    if (views.first.id === 1) {
+      return true;
+    } else if (views.last.id === this._thumbnails.length) {
+      return false;
+    }
+    return this.scroll.down;
+  }
+
   forceRendering() {
     const visibleThumbs = this._getVisibleThumbs();
+    const scrollAhead = this.#getScrollAhead(visibleThumbs);
     const thumbView = this.renderingQueue.getHighestPriority(
       visibleThumbs,
       this._thumbnails,
-      this.scroll.down
+      scrollAhead
     );
     if (thumbView) {
       this._ensurePdfPageLoaded(thumbView).then(() => {