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.

web/xfa_layer_builder.css

@@ -13,6 +13,15 @@
  * limitations under the License.
  */
 
+.xfaPage {
+  overflow: hidden;
+  position: relative;
+}
+
+.xfaContentarea {
+  position: absolute;
+}
+
 .xfaPrintOnly {
   display: none;
 }
@@ -139,10 +148,6 @@
   flex: 1 1 auto;
 }
 
-.xfaContentArea {
-  overflow: hidden;
-}
-
 .xfaTextfield,
 .xfaSelect {
   background-color: rgba(0, 54, 255, 0.13);