Ensure that cleanup during rendering is actually ignored, to prevent a blank canvas

The existing unit-test doesn't work as intended, since the page never actually renders. Note how `cleanup` is *not* allowed to run when parsing and/or rendering is ongoing, however an (old) incorrect condition could prevent rendering from ever starting.

This is very old code, which has been slightly re-factored a couple of times (many years ago), however this doesn't appear to affect e.g. the default viewer since the incorrect behaviour seem highly dependent on "unlucky" timing.
Note also how at the start of the `PDFPageProxy.prototype.render`-method we purposely cancel any pending `cleanup`-call, to prevent unnecessary re-parsing for multiple sequential `render`-calls.

Finally, avoid running `cleanup` when document/page destruction has already started since it's pointless in that case.

src/display/api.js

@@ -1524,7 +1524,7 @@ class PDFPageProxy {
       optionalContentConfigPromise,
     ])
       .then(([transparency, optionalContentConfig]) => {
-        if (this.#pendingCleanup) {
+        if (this.destroyed) {
           complete();
           return;
         }
@@ -1727,7 +1727,7 @@ class PDFPageProxy {
   #tryCleanup(delayed = false) {
     this.#abortDelayedCleanup();
 
-    if (!this.#pendingCleanup) {
+    if (!this.#pendingCleanup || this.destroyed) {
       return false;
     }
     if (delayed) {

test/unit/api_spec.js

@@ -3204,6 +3204,7 @@ Caron Broadcasting, Inc., an Ohio corporation (“Lessee”).`)
       const renderTask = pdfPage.render({
         canvasContext: canvasAndCtx.context,
         viewport,
+        background: "#FF0000", // See comment below.
       });
       expect(renderTask instanceof RenderTask).toEqual(true);
 
@@ -3226,6 +3227,11 @@ Caron Broadcasting, Inc., an Ohio corporation (“Lessee”).`)
       await renderTask.promise;
       expect(renderTask.separateAnnots).toEqual(false);
 
+      // Use the red background-color to, more easily, tell that the page was
+      // actually rendered successfully.
+      const { data } = canvasAndCtx.context.getImageData(0, 0, 1, 1);
+      expect(data).toEqual(new Uint8ClampedArray([255, 0, 0, 255]));
+
       CanvasFactory.destroy(canvasAndCtx);
       await loadingTask.destroy();
     });