Check if `pageView.pdfPage` exists in `PDFViewer._ensurePdfPageLoaded`, to avoid potentially calling `PDFPageView.setPdfPage` multiple times for the same page

Since calling `PDFPageView.setPdfPage` will in turn call `PDFPageView.reset`, which cancels all rendering and completely resets the page, it's thus possible that we currently cause some unnecessary re-rendering during the initial loading phase of the viewer.

Depending on the order in which data arrives, it's possible (and in practice always seem to happen) that the `pdfPage` property of the *second* page has already been set during `PDFViewer.setDocument`, by the time that the request for the `pdfPage` is resolved in `PDFViewer._ensurePdfPageLoaded`.

Also, note how the `setPdfPage` call in `PDFViewer.setDocument` is already guarded by this kind of check.

web/pdf_viewer.js

@@ -843,12 +843,13 @@ var PDFViewer = (function pdfViewer() {
       if (this._pagesRequests[pageNumber]) {
         return this._pagesRequests[pageNumber];
       }
-      var promise = this.pdfDocument.getPage(pageNumber).then(
-          function (pdfPage) {
+      var promise = this.pdfDocument.getPage(pageNumber).then((pdfPage) => {
+        if (!pageView.pdfPage) {
           pageView.setPdfPage(pdfPage);
+        }
         this._pagesRequests[pageNumber] = null;
         return pdfPage;
-      }.bind(this));
+      });
       this._pagesRequests[pageNumber] = promise;
       return promise;
     },
@@ -859,9 +860,9 @@ var PDFViewer = (function pdfViewer() {
                                                             this._pages,
                                                             this.scroll.down);
       if (pageView) {
-        this._ensurePdfPageLoaded(pageView).then(function () {
+        this._ensurePdfPageLoaded(pageView).then(() => {
           this.renderingQueue.renderView(pageView);
-        }.bind(this));
+        });
         return true;
       }
       return false;