Implement missing XForm.Finish() method

Author: ascott18

PdfSharpCore/Drawing/XForm.cs

@@ -177,6 +177,28 @@ protected override void Dispose(bool disposing)
         /// </summary>
         internal virtual void Finish()
         {
+            if (_formState == FormState.NotATemplate || _formState == FormState.Finished)
+                return;
+
+            Debug.Assert(_formState == FormState.Created || _formState == FormState.UnderConstruction);
+            _formState = FormState.Finished;
+            Gfx.Dispose();
+            Gfx = null;
+
+            if (PdfRenderer != null)
+            {
+                //pdfForm.CreateStream(PdfEncoders.RawEncoding.GetBytes(PdfRenderer.GetContent()));
+                PdfRenderer.Close();
+                Debug.Assert(PdfRenderer == null);
+
+                if (_document.Options.CompressContentStreams)
+                {
+                    _pdfForm.Stream.Value = Filtering.FlateDecode.Encode(_pdfForm.Stream.Value, _document.Options.FlateEncodeMode);
+                    _pdfForm.Elements["/Filter"] = new PdfName("/FlateDecode");
+                }
+                int length = _pdfForm.Stream.Length;
+                _pdfForm.Elements.SetInteger("/Length", length);
+            }
         }
 
         /// <summary>
@@ -437,4 +459,4 @@ string IContentStream.GetFormName(XForm form)
         internal XGraphicsPdfRenderer PdfRenderer;
 
     }
-}
+}