[PDFThumbnailView] Re-factor the canvas to image conversion such that we always render to a canvas, and then replace it with an image once rendering is done

*This is a follow-up to PRs 6299 and 6441.*

The patch also adds an option to `PDFThumbnailView`, that disables the canvas-to-image conversion entirely, which might be useful in the context of issue 7026.

web/pdf_thumbnail_viewer.js

@@ -147,7 +147,8 @@ var PDFThumbnailViewer = (function PDFThumbnailViewerClosure() {
             id: pageNum,
             defaultViewport: viewport.clone(),
             linkService: this.linkService,
-            renderingQueue: this.renderingQueue
+            renderingQueue: this.renderingQueue,
+            disableCanvasToImageConversion: false,
           });
           this.thumbnails.push(thumbnail);
         }