XFA - Remove empty pages

- it aims to fix #13583;
  - fix the switch to breakBefore target;
  - force the layout of an unsplittable element on an empty page;
  - don't fail when there is horizontal overflow (except in lr-tb);
  - handle correctly overflow in the same content area (bug 1717805, bug 1717668);
  - fix a typo in radial gradient first argument.

src/core/xfa/xfa_object.js

@@ -43,6 +43,7 @@ const $getChildrenByClass = Symbol();
 const $getChildrenByName = Symbol();
 const $getChildrenByNameIt = Symbol();
 const $getDataValue = Symbol();
+const $getExtra = Symbol();
 const $getRealChildrenByNameIt = Symbol();
 const $getChildren = Symbol();
 const $getContainedChildren = Symbol();
@@ -177,11 +178,7 @@ class XFAObject {
   }
 
   [$getTemplateRoot]() {
-    let parent = this[$getParent]();
-    while (parent[$nodeName] !== "template") {
-      parent = parent[$getParent]();
-    }
-    return parent;
+    return this[$globalData].template;
   }
 
   [$isSplittable]() {
@@ -1060,6 +1057,7 @@ export {
   $getChildrenByNameIt,
   $getContainedChildren,
   $getDataValue,
+  $getExtra,
   $getNextPage,
   $getParent,
   $getRealChildrenByNameIt,