Add a pref to know if the alt-text must be automatically generated (bug 1909310)

web/app.js

@@ -205,10 +205,10 @@ const PDFViewerApplication = {
       if (mode) {
         document.documentElement.classList.add(mode);
       }
-    } else {
+    } else if (AppOptions.get("enableAltText")) {
       // We want to load the image-to-text AI engine as soon as possible.
       this.mlManager = new MLManager({
-        enableAltText: AppOptions.get("enableAltText"),
+        enableGuessAltText: AppOptions.get("enableGuessAltText"),
         altTextLearnMoreUrl: AppOptions.get("altTextLearnMoreUrl"),
       });
     }
@@ -376,14 +376,14 @@ const PDFViewerApplication = {
 
     let eventBus;
     if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) {
-      eventBus =
-        AppOptions.eventBus =
-        this.mlManager.eventBus =
-          new FirefoxEventBus(
-            AppOptions.get("allowedGlobalEvents"),
-            externalServices,
-            AppOptions.get("isInAutomation")
-          );
+      eventBus = AppOptions.eventBus = new FirefoxEventBus(
+        AppOptions.get("allowedGlobalEvents"),
+        externalServices,
+        AppOptions.get("isInAutomation")
+      );
+      if (this.mlManager) {
+        this.mlManager.eventBus = eventBus;
+      }
     } else {
       eventBus = new EventBus();
     }

web/app_options.js

@@ -179,6 +179,11 @@ const defaultOptions = {
     value: false,
     kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
   },
+  enableGuessAltText: {
+    /** @type {boolean} */
+    value: true,
+    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
+  },
   enableHighlightEditor: {
     // We'll probably want to make some experiments before enabling this
     // in Firefox release, but it has to be temporary.

web/firefoxcom.js

@@ -328,8 +328,8 @@ class MLManager {
     return FirefoxCom.requestAsync("mlGuess", data);
   }
 
-  enable({ altTextLearnMoreUrl, enableAltText, listenToProgress }) {
-    if (enableAltText) {
+  enable({ altTextLearnMoreUrl, enableGuessAltText, listenToProgress }) {
+    if (enableGuessAltText) {
       this.#loadAltTextEngine(listenToProgress);
     }
     // The `altTextLearnMoreUrl` is used to provide a link to the user to learn