Add a InternalRenderTask.completed getter and use it to simplify PDFPageProxy._destroy

This patch aims to simplify the `PDFPageProxy._destroy` method, by:
 - Replacing the unnecessary `forEach` with a "regular" `for`-loop instead.
 - Use a more appropriate variable name, since `intentState.renderTasks` contain instances of `InternalRenderTask`.
 - Move the "is rendering completed"-handling to a new `InternalRenderTask.completed` getter, to abstract away some (mostly) internal `InternalRenderTask` state.

src/display/api.js

@@ -1234,13 +1234,10 @@ class PDFPageProxy {
         // Avoid errors below, since the renderTasks are just stubs.
         return;
       }
-      intentState.renderTasks.forEach(function (renderTask) {
+      for (const internalRenderTask of intentState.renderTasks) {
-        const renderCompleted = renderTask.capability.promise.catch(
+        waitOn.push(internalRenderTask.completed);
-          function () {}
+        internalRenderTask.cancel();
-        ); // ignoring failures
+      }
-        waitOn.push(renderCompleted);
-        renderTask.cancel();
-      });
     });
     this.objs.clear();
     this.annotationsPromise = null;
@@ -2650,6 +2647,13 @@ const InternalRenderTask = (function InternalRenderTaskClosure() {
       this._canvas = params.canvasContext.canvas;
     }
 
+    get completed() {
+      return this.capability.promise.catch(function () {
+        // Ignoring errors, since we only want to know when rendering is
+        // no longer pending.
+      });
+    }
+
     initializeGraphics(transparency = false) {
       if (this.cancelled) {
         return;