Call FPDF_LoadXFA() in init_forms(), warn on error

FPDF_LoadXFA() currently seems to be broken in pdfium-binaries.
All the same, add the call so that pypdfium2 is prepared for an
eventual upstream fix.

Currently this only warns on error. Once the aforementioned issue
is fixed, it should be tightened to a PdfiumError exception.

See also #221 #222

src/pypdfium2/_helpers/document.py

@@ -146,18 +146,36 @@ def init_forms(self, config=None):
             config (FPDF_FORMFILLINFO | None):
                 Custom form config interface to use (optional).
         """
-        if (self.get_formtype() == pdfium_c.FORMTYPE_NONE) or self.formenv:
+
+        formtype = self.get_formtype()
+        if formtype == pdfium_c.FORMTYPE_NONE or self.formenv:
             return
+
         if not config:
+
+            # safety check for older binaries (could be removed at some point)
+            # https://github.com/bblanchon/pdfium-binaries/issues/105
             if V_PDFIUM_IS_V8 and V_BUILDNAME == "pdfium-binaries" and int(V_LIBPDFIUM) <= 5677:
-                # https://github.com/bblanchon/pdfium-binaries/issues/105
                 raise RuntimeError("V8 enabled pdfium-binaries builds <= 5677 crash on init_forms().")
-            config = pdfium_c.FPDF_FORMFILLINFO(version=2)
+
+            if V_PDFIUM_IS_V8:
+                js_platform = pdfium_c.IPDF_JSPLATFORM(version=3)
+                config = pdfium_c.FPDF_FORMFILLINFO(version=2, xfa_disabled=False, jsPlatform=js_platform)
+            else:
+                config = pdfium_c.FPDF_FORMFILLINFO(version=2)
+
         raw = pdfium_c.FPDFDOC_InitFormFillEnvironment(self, config)
         if not raw:
             raise PdfiumError(f"Initializing form env failed for document {self}.")
         self.formenv = PdfFormEnv(raw, config, self)
-        self._add_kid(self.formenv)  # guarantees correct order of closing
+        self._add_kid(self.formenv)
+
+        if V_PDFIUM_IS_V8 and formtype in (pdfium_c.FORMTYPE_XFA_FOREGROUND, pdfium_c.FORMTYPE_XFA_FULL):
+            ok = pdfium_c.FPDF_LoadXFA(self)
+            if not ok:
+                # always fails as of this writing, thus warn instead of raising an exception for now
+                # probably this is due to an issue with pdfium-binaries - once fixed, this may be tightened to an exception
+                logger.warning(f"Failed to load XFA for document {self}.")
 
 
     # TODO?(v5) consider cached property