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.

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(() => {