Pre-render *one* additional page when spreadModes are enabled

Please note that we (obviously) don't want to unconditionally pre-render more than one page all the time, since that could very easily lead to overall worse performance in some documents.[1]
However, when spreadModes are enabled it does make sense to attempt to pre-render both of the pages of the next/previous spread.

---
[1] Since it may cause pre-rendering to unnecessarily compete for parsing resources, on the worker-thread, with "regular" rendering.

Author: Snuffleupagus

web/base_viewer.js

@@ -1250,10 +1250,16 @@ class BaseViewer {
     const scrollAhead = this._isScrollModeHorizontal
       ? this.scroll.right
       : this.scroll.down;
+    const preRenderExtra =
+      this._scrollMode === ScrollMode.VERTICAL &&
+      this._spreadMode !== SpreadMode.NONE &&
+      !this.isInPresentationMode;
+
     const pageView = this.renderingQueue.getHighestPriority(
       visiblePages,
       this._pages,
-      scrollAhead
+      scrollAhead,
+      preRenderExtra
     );
     if (pageView) {
       this._ensurePdfPageLoaded(pageView).then(() => {

web/pdf_rendering_queue.js

@@ -102,8 +102,9 @@ class PDFRenderingQueue {
    * @param {Object} visible
    * @param {Array} views
    * @param {boolean} scrolledDown
+   * @param {boolean} [preRenderExtra]
    */
-  getHighestPriority(visible, views, scrolledDown) {
+  getHighestPriority(visible, views, scrolledDown, preRenderExtra = false) {
     /**
      * The state has changed. Figure out which page has the highest priority to
      * render next (if any).
@@ -128,14 +129,20 @@ class PDFRenderingQueue {
 
     // All the visible views have rendered; try to render next/previous page.
     // (IDs start at 1, so no need to add 1 when `scrolledDown === true`.)
-    const preRenderIndex = scrolledDown
-      ? visible.last.id
-      : visible.first.id - 2;
-    const preRenderView = views[preRenderIndex];
+    let preRenderIndex = scrolledDown ? visible.last.id : visible.first.id - 2;
+    let preRenderView = views[preRenderIndex];
 
     if (preRenderView && !this.isViewFinished(preRenderView)) {
       return preRenderView;
     }
+    if (preRenderExtra) {
+      preRenderIndex += scrolledDown ? 1 : -1;
+      preRenderView = views[preRenderIndex];
+
+      if (preRenderView && !this.isViewFinished(preRenderView)) {
+        return preRenderView;
+      }
+    }
     // Everything that needs to be rendered has been.
     return null;
   }