Dispatch a 'pagecancelled' event, in PDFPageView.cancelRendering, when rendering is cancelled

Also, the patch updates `TextLayerBuilder` to use the new 'pagecancelled' event for (future) event removal purposes.

web/pdf_page_view.js

@@ -260,6 +260,8 @@ class PDFPageView {
   }
 
   cancelRendering(keepAnnotations = false) {
+    const renderingState = this.renderingState;
+
     if (this.paintTask) {
       this.paintTask.cancel();
       this.paintTask = null;
@@ -275,6 +277,14 @@ class PDFPageView {
       this.annotationLayer.cancel();
       this.annotationLayer = null;
     }
+
+    if (renderingState !== RenderingStates.INITIAL) {
+      this.eventBus.dispatch('pagecancelled', {
+        source: this,
+        pageNumber: this.id,
+        renderingState,
+      });
+    }
   }
 
   cssTransform(target, redrawAnnotations = false) {

web/text_layer_builder.js

@@ -56,6 +56,9 @@ class TextLayerBuilder {
     this.textLayerRenderTask = null;
     this.enhanceTextSelection = enhanceTextSelection;
 
+    this._boundEvents = Object.create(null);
+    this._bindEvents();
+
     this._bindMouse();
   }
 
@@ -331,6 +334,33 @@ class TextLayerBuilder {
     this.renderMatches(this.matches);
   }
 
+  /**
+   * @private
+   */
+  _bindEvents() {
+    const { eventBus, _boundEvents, } = this;
+
+    _boundEvents.pageCancelled = (evt) => {
+      if (evt.pageNumber !== this.pageNumber) {
+        return;
+      }
+      if (this.textLayerRenderTask) {
+        console.error('TextLayerBuilder._bindEvents: `this.cancel()` should ' +
+          'have been called when the page was reset, or rendering cancelled.');
+        return;
+      }
+      // Ensure that all event listeners are cleaned up when the page is reset,
+      // since re-rendering will create new `TextLayerBuilder` instances and the
+      // number of (stale) event listeners would otherwise grow without bound.
+      for (const name in _boundEvents) {
+        eventBus.off(name.toLowerCase(), _boundEvents[name]);
+        delete _boundEvents[name];
+      }
+    };
+
+    eventBus.on('pagecancelled', _boundEvents.pageCancelled);
+  }
+
   /**
    * Improves text selection by adding an additional div where the mouse was
    * clicked. This reduces flickering of the content if the mouse is slowly