Update the Annotation --zoom-factor CSS variable when PDFPageView is used standalone (PR 13868 follow-up)

Without this patch, when using `PDFPageView` directly[1] this CSS variable won't be updated and consequently things won't work as intended.
This is purposely implemented such that when a `PDFPageView`-instance is part of a viewer, we don't repeatedly set the CSS variable for every single page.

---
[1] See e.g. the "pageviewer" example in the `examples/components/` folder.

web/pdf_rendering_queue.js

@@ -60,6 +60,13 @@ class PDFRenderingQueue {
     return this.highestPriorityPage === view.renderingId;
   }
 
+  /**
+   * @returns {boolean}
+   */
+  hasViewer() {
+    return !!this.pdfViewer;
+  }
+
   /**
    * @param {Object} currentlyVisiblePages
    */