[api-minor] Improve thumbnail handling in documents that contain interactive forms

To improve performance of the sidebar we use the page-canvases to generate the thumbnails whenever possible, since that avoids unnecessary re-rendering when the sidebar is open. This works generally well, however there's an old problem in PDF documents that contain interactive forms (when those are enabled): Note how the thumbnails become partially (or fully) blank, since those Annotations are not included in the OperatorList.[1]

We obviously want to keep using the `PDFThumbnailView.setImage`-method for most documents, however we need a way to skip it only for those pages that contain interactive forms.
As it turns out it's unfortunately not all that simple to tell, after the fact, from looking only at the OperatorList that some Annotations were skipped. While it might have been possible to try and infer that in the viewer, it'd not have been pretty considering that at the time when rendering finishes the annotationLayer has not yet been built.
The overall simplest solution that I could come up with, was instead to include a *summary* of the interactive form-state when doing the final "flushing" of the OperatorList and expose that information in the API.

---
[1] Some examples from our test-suite: `annotation-tx2.pdf` where the thumbnail is completely blank, and `bug1737260.pdf` where the thumbnail is missing the "buttons" found on the page.

web/pdf_page_view.js

@@ -99,7 +99,10 @@ const MAX_CANVAS_PIXELS = compatibilityParams.maxCanvasPixels || 16777216;
 class PDFPageView {
   #annotationMode = AnnotationMode.ENABLE_FORMS;
 
-  #useThumbnailCanvas = true;
+  #useThumbnailCanvas = {
+    initialOptionalContent: true,
+    regularAnnotations: true,
+  };
 
   /**
    * @param {PDFPageViewOptions} options
@@ -190,14 +193,15 @@ class PDFPageView {
       const { optionalContentConfigPromise } = options;
       if (optionalContentConfigPromise) {
         // Ensure that the thumbnails always display the *initial* document
-        // state.
+        // state, for documents with optional content.
         optionalContentConfigPromise.then(optionalContentConfig => {
           if (
             optionalContentConfigPromise !== this._optionalContentConfigPromise
           ) {
             return;
           }
-          this.#useThumbnailCanvas = optionalContentConfig.hasInitialVisibility;
+          this.#useThumbnailCanvas.initialOptionalContent =
+            optionalContentConfig.hasInitialVisibility;
         });
       }
     }
@@ -408,14 +412,16 @@ class PDFPageView {
     if (optionalContentConfigPromise instanceof Promise) {
       this._optionalContentConfigPromise = optionalContentConfigPromise;
 
-      // Ensure that the thumbnails always display the *initial* document state.
+      // Ensure that the thumbnails always display the *initial* document state,
+      // for documents with optional content.
       optionalContentConfigPromise.then(optionalContentConfig => {
         if (
           optionalContentConfigPromise !== this._optionalContentConfigPromise
         ) {
           return;
         }
-        this.#useThumbnailCanvas = optionalContentConfig.hasInitialVisibility;
+        this.#useThumbnailCanvas.initialOptionalContent =
+          optionalContentConfig.hasInitialVisibility;
       });
     }
 
@@ -772,6 +778,10 @@ class PDFPageView {
       }
       this._resetZoomLayer(/* removeFromDOM = */ true);
 
+      // Ensure that the thumbnails won't become partially (or fully) blank,
+      // for documents that contain interactive form elements.
+      this.#useThumbnailCanvas.regularAnnotations = !paintTask.separateAnnots;
+
       this.eventBus.dispatch("pagerendered", {
         source: this,
         pageNumber: this.id,
@@ -888,6 +898,9 @@ class PDFPageView {
       cancel() {
         renderTask.cancel();
       },
+      get separateAnnots() {
+        return renderTask.separateAnnots;
+      },
     };
 
     const viewport = this.viewport;
@@ -1029,6 +1042,9 @@ class PDFPageView {
       cancel() {
         cancelled = true;
       },
+      get separateAnnots() {
+        return false;
+      },
     };
   }
 
@@ -1050,7 +1066,9 @@ class PDFPageView {
    * @ignore
    */
   get thumbnailCanvas() {
-    return this.#useThumbnailCanvas ? this.canvas : null;
+    const { initialOptionalContent, regularAnnotations } =
+      this.#useThumbnailCanvas;
+    return initialOptionalContent && regularAnnotations ? this.canvas : null;
   }
 }