Display widget signature

- but don't validate them for now;
  - Firefox will display a bar to warn that the signature validation is not supported (see https://bugzilla.mozilla.org/show_bug.cgi?id=854315)
  - almost all (all ?) pdf readers display signatures;
  - validation is done in edge but for now it's behind a pref.

src/core/annotation.js

@@ -126,6 +126,8 @@ class AnnotationFactory {
             return new ButtonWidgetAnnotation(parameters);
           case "Ch":
             return new ChoiceWidgetAnnotation(parameters);
+          case "Sig":
+            return new SignatureWidgetAnnotation(parameters);
         }
         warn(
           `Unimplemented widget field type "${fieldType}", ` +
@@ -1151,15 +1153,6 @@ class WidgetAnnotation extends Annotation {
 
     data.readOnly = this.hasFieldFlag(AnnotationFieldFlag.READONLY);
     data.hidden = this._hasFlag(data.annotationFlags, AnnotationFlag.HIDDEN);
-
-    // Hide signatures because we cannot validate them, and unset the fieldValue
-    // since it's (most likely) a `Dict` which is non-serializable and will thus
-    // cause errors when sending annotations to the main-thread (issue 10347).
-    if (data.fieldType === "Sig") {
-      data.fieldValue = null;
-      this.setFlags(AnnotationFlag.HIDDEN);
-      data.hidden = true;
-    }
   }
 
   /**
@@ -1201,7 +1194,7 @@ class WidgetAnnotation extends Annotation {
   getOperatorList(evaluator, task, renderForms, annotationStorage) {
     // Do not render form elements on the canvas when interactive forms are
     // enabled. The display layer is responsible for rendering them instead.
-    if (renderForms) {
+    if (renderForms && !(this instanceof SignatureWidgetAnnotation)) {
       return Promise.resolve(new OperatorList());
     }
 
@@ -1600,13 +1593,6 @@ class WidgetAnnotation extends Annotation {
   }
 
   getFieldObject() {
-    if (this.data.fieldType === "Sig") {
-      return {
-        id: this.data.id,
-        value: null,
-        type: "signature",
-      };
-    }
     return null;
   }
 }
@@ -2203,6 +2189,25 @@ class ChoiceWidgetAnnotation extends WidgetAnnotation {
   }
 }
 
+class SignatureWidgetAnnotation extends WidgetAnnotation {
+  constructor(params) {
+    super(params);
+
+    // Unset the fieldValue since it's (most likely) a `Dict` which is
+    // non-serializable and will thus cause errors when sending annotations
+    // to the main-thread (issue 10347).
+    this.data.fieldValue = null;
+  }
+
+  getFieldObject() {
+    return {
+      id: this.data.id,
+      value: null,
+      type: "signature",
+    };
+  }
+}
+
 class TextAnnotation extends MarkupAnnotation {
   constructor(parameters) {
     const DEFAULT_ICON_SIZE = 22; // px