XFA - Fix a breakBefore issue when target is a contentArea and startNew is 1 (bug 1739502)

- it aims to fix https://bugzilla.mozilla.org/show_bug.cgi?id=1739502;
 - when the target area was the current content area, everything was pushed in it instead of creating a new one (and consequently a new pageArea is created).
 - the pdf shows an alignment issue on page 4:
   - the hAlign is "center" but the subform was the width of its parent, so compute the real width of the subform with tb layout;
 - there is an extra empty page at the end of the pdf:
   - there is a subform with some hidden elements which are not rendered for now (since there is no plugged JS engine it isn't possible to draw them in changing their visibility).
   - so in case a subform is empty and has no real dimensions (at least one is 0), we just consider it as empty.

test/pdfs/xfa_bug1739502.pdf.link

@@ -0,0 +1 @@
+https://bugzilla.mozilla.org/attachment.cgi?id=9249303

test/test_manifest.json

@@ -1033,6 +1033,14 @@
        "enableXfa": true,
        "type": "eq"
     },
+    {  "id": "xfa_bug1739502",
+       "file": "pdfs/xfa_bug1739502.pdf",
+       "md5": "a760a950cb225c3cbf25a55b5d95e264",
+       "link": true,
+       "rounds": 1,
+       "enableXfa": true,
+       "type": "eq"
+    },
     {  "id": "xfa_bug1729877",
        "file": "pdfs/xfa_bug1729877.pdf",
        "md5": "1bf5429ea13a090f52edcd49cb0c4a47",