Prevent the annotationLayer from, in some cases, becoming duplicated on the first page when the document loads

I don't know if this is a regression, but I noticed earlier today that depending on the initial scale *and* sidebar state, the `annotationLayer` of the first rendered page may end up duplicated; please see screen-shot below.

[screen-shot]

I can reproduce this reliable with e.g. https://arxiv.org/pdf/1112.0542v1.pdf#zoom=page-width&pagemode=bookmarks.
When the document loads, rendering of the first page begins immediately. When the sidebar is then opened, that forces re-rendering which thus aborts rendering of the first page.
Note that calling `PDFPageView.draw()` will always, provided an `AnnotationLayerFactory` instance exists, call `AnnotationLayerBuilder.render()`. Hence the events described above will result in *two* such calls, where the actual annotation rendering/updating happens asynchronously.

For reasons that I don't (at all) understand, when multiple `pdfPage.getAnnotations()` promises are handled back-to-back (in `AnnotationLayerBuilder.render()`), the `this.div` property seems to not update in time for the subsequent calls.
This thus, at least in Firefox, result in double rendering of all annotations on the first page.

Obviously it'd be good to find out why it breaks, since it *really* shouldn't, but this patch at least provides a (hopefully) acceptable work-around by ignoring `getAnnotations()` calls for `AnnotationLayerBuilder` instances that we're destroying (in `PDFPageView.reset()`).

web/annotation_layer_builder.js

@@ -41,6 +41,7 @@ class AnnotationLayerBuilder {
     this.l10n = l10n;
 
     this.div = null;
+    this._cancelled = false;
   }
 
   /**
@@ -49,6 +50,10 @@ class AnnotationLayerBuilder {
    */
   render(viewport, intent = 'display') {
     this.pdfPage.getAnnotations({ intent, }).then((annotations) => {
+      if (this._cancelled) {
+        return;
+      }
+
       let parameters = {
         viewport: viewport.clone({ dontFlip: true, }),
         div: this.div,
@@ -80,6 +85,10 @@ class AnnotationLayerBuilder {
     });
   }
 
+  cancel() {
+    this._cancelled = true;
+  }
+
   hide() {
     if (!this.div) {
       return;

web/pdf_page_view.js

@@ -136,7 +136,7 @@ class PDFPageView {
   }
 
   reset(keepZoomLayer = false, keepAnnotations = false) {
-    this.cancelRendering();
+    this.cancelRendering(keepAnnotations);
 
     let div = this.div;
     div.style.width = Math.floor(this.viewport.width) + 'px';
@@ -159,7 +159,8 @@ class PDFPageView {
       // Hide the annotation layer until all elements are resized
       // so they are not displayed on the already resized page.
       this.annotationLayer.hide();
-    } else {
+    } else if (this.annotationLayer) {
+      this.annotationLayer.cancel();
       this.annotationLayer = null;
     }
 
@@ -240,7 +241,7 @@ class PDFPageView {
     this.reset(/* keepZoomLayer = */ true, /* keepAnnotations = */ true);
   }
 
-  cancelRendering() {
+  cancelRendering(keepAnnotations = false) {
     if (this.paintTask) {
       this.paintTask.cancel();
       this.paintTask = null;
@@ -252,6 +253,10 @@ class PDFPageView {
       this.textLayer.cancel();
       this.textLayer = null;
     }
+    if (!keepAnnotations && this.annotationLayer) {
+      this.annotationLayer.cancel();
+      this.annotationLayer = null;
+    }
   }
 
   cssTransform(target, redrawAnnotations = false) {