Proof of concept formenv segfault

If externalising the form fill environment to document level, the exit
call segfaults.

src/pypdfium2/_helpers/document.py

@@ -78,6 +78,7 @@ def __init__(
         self._actual_input = input_data
         self._rendering_input = None
         self._ld_data = None
+        self._form_env = None
 
         self._password = password
         self._file_access = file_access
@@ -143,13 +144,31 @@ def close(self):
         Close the document to release allocated memory.
         This function shall be called when finished working with the object.
         """
+        if self._form_env is not None:
+            print(self._form_env)
+            pdfium.FPDFDOC_ExitFormFillEnvironment(self._form_env)  # this call segfaults
         pdfium.FPDF_CloseDocument(self.raw)
         if self._ld_data is not None:
             self._ld_data.close()
         if self._autoclose and is_input_buffer(self._actual_input):
             self._actual_input.close()
 
 
+    def init_formenv(self):
+        """
+        Initialise a form environment object for this document.
+        If already initialised, the existing one will be returned instead.
+        :meth:`.close` will free the form environment, if initialised.
+        
+        Returns:
+            FPDF_FORMHANDLE:
+        """
+        if self._form_env is None:
+            form_info = pdfium.FPDF_FORMFILLINFO(1)
+            self._form_env = pdfium.FPDFDOC_InitFormFillEnvironment(self.raw, form_info)
+        return self._form_env
+
+
     def get_version(self):
         """
         Returns:

src/pypdfium2/_helpers/page.py

@@ -422,10 +422,8 @@ def render_base(
             pdfium.FPDF_RenderPage_Close(self.raw)
 
         if draw_forms:
-            form_info = pdfium.FPDF_FORMFILLINFO(1)
-            form_env = pdfium.FPDFDOC_InitFormFillEnvironment(self.pdf.raw, form_info)
+            form_env = self.pdf.init_formenv()
             pdfium.FPDF_FFLDraw(form_env, *render_args)
-            pdfium.FPDFDOC_ExitFormFillEnvironment(form_env)
 
         return buffer, cl_string, (width, height)