XFA - Get line height from the font

- when the CSS line-height property is set to 'normal' then the value depends of the user agent. So use a line height based on the font itself and if for any reasons this value is not available use 1.2 as default.
  - it's a partial fix for https://bugzilla.mozilla.org/show_bug.cgi?id=1717681.

test/pdfs/xfa_bug1717681.pdf.link

@@ -0,0 +1 @@
+https://bugzilla.mozilla.org/attachment.cgi?id=9228400

test/test_manifest.json

@@ -930,6 +930,14 @@
        "link": true,
        "type": "load"
     },
+    {  "id": "xfa_bug1717681",
+       "file": "pdfs/xfa_bug1717681.pdf",
+       "md5": "435b1eae7e017b1a932fe204d1ba8be5",
+       "link": true,
+       "rounds": 1,
+       "enableXfa": true,
+       "type": "eq"
+    },
     {  "id": "xfa_bug1716380",
        "file": "pdfs/xfa_bug1716380.pdf",
        "md5": "1351f816f0509fe750ca61ef2bd40872",