Fix re-rendering, using the same canvas, when rendering was previously cancelled (PR 8519 follow-up)

Currently if `RenderTask.cancel` is called *immediately* after rendering was started, then by the time that `InternalRenderTask.initializeGraphics` is called rendering will already have been cancelled.
However, we're still inserting the canvas into the `canvasInRendering` map, thus breaking any future attempts at re-rendering using the same canvas. Considering that `InternalRenderTask.cancel` always removes the canvas from the map, I cannot imagine that we'd ever want to re-add it *after* rendering was cancelled (it was likely just a simple oversight in PR 8519).

Fixes 9456.

test/unit/api_spec.js

@@ -1253,6 +1253,37 @@ describe('api', function() {
         done();
       });
     });
+
+    it('re-render page, using the same canvas, after cancelling rendering',
+        function(done) {
+      if (isNodeJS()) {
+        pending('TODO: Support Canvas testing in Node.js.');
+      }
+      let viewport = page.getViewport(1);
+      let canvasAndCtx = CanvasFactory.create(viewport.width, viewport.height);
+
+      let renderTask = page.render({
+        canvasContext: canvasAndCtx.context,
+        viewport,
+      });
+      renderTask.cancel();
+
+      renderTask.promise.then(() => {
+        throw new Error('shall cancel rendering');
+      }, (reason) => {
+        expect(reason instanceof RenderingCancelledException).toEqual(true);
+      }).then(() => {
+        let reRenderTask = page.render({
+          canvasContext: canvasAndCtx.context,
+          viewport,
+        });
+        return reRenderTask.promise;
+      }).then(() => {
+        CanvasFactory.destroy(canvasAndCtx);
+        done();
+      }, done.fail);
+    });
+
     it('multiple render() on the same canvas', function(done) {
       if (isNodeJS()) {
         pending('TODO: Support Canvas testing in Node.js.');