Reinitialize form field appearance stream object on every field regeneration

First, appearance stream objects might be shared between different forms fields or other entities, if they are identical, so on regeneration we should not affect those other entities.

Second, the resources of the streams and other entries in stream dictionaries are not shared between regenerations. It seems that this makes sense, because resources are completely rebuilt every time, and all other important entries in the appearance stream are updated on regeneration as well.

DEVSIX-2374

Autoported commit.
Original commit hash: [70def8325]

Author: yulian-gaponenko

itext.tests/itext.forms.tests/itext/forms/PdfFormCopyTest.cs

@@ -237,6 +237,31 @@ public virtual void CopyMultipleSubfieldsTest03() {
                 , destinationFolder, "diff_"));
         }
 
+        /// <exception cref="System.IO.IOException"/>
+        /// <exception cref="System.Exception"/>
+        [NUnit.Framework.Test]
+        [LogMessage(iText.IO.LogMessageConstant.DOCUMENT_ALREADY_HAS_FIELD, Count = 12)]
+        public virtual void CopyMultipleSubfieldsSmartModeTest01() {
+            String srcFilename = sourceFolder + "copyMultipleSubfieldsSmartModeTest01.pdf";
+            String destFilename = destinationFolder + "copyMultipleSubfieldsSmartModeTest01.pdf";
+            PdfDocument srcDoc = new PdfDocument(new PdfReader(srcFilename));
+            PdfDocument destDoc = new PdfDocument(new PdfWriter(destFilename).SetSmartMode(true));
+            PdfPageFormCopier pdfPageFormCopier = new PdfPageFormCopier();
+            // copying the same page from the same document twice
+            for (int i = 0; i < 4; ++i) {
+                srcDoc.CopyPagesTo(1, 1, destDoc, pdfPageFormCopier);
+            }
+            PdfAcroForm acroForm = PdfAcroForm.GetAcroForm(destDoc, false);
+            acroForm.GetField("text_1").SetValue("Text 1!");
+            acroForm.GetField("text_2").SetValue("Text 2!");
+            acroForm.GetField("text.3").SetValue("Text 3!");
+            acroForm.GetField("text.4").SetValue("Text 4!");
+            destDoc.Close();
+            srcDoc.Close();
+            NUnit.Framework.Assert.IsNull(new CompareTool().CompareByContent(destFilename, sourceFolder + "cmp_copyMultipleSubfieldsSmartModeTest01.pdf"
+                , destinationFolder, "diff_"));
+        }
+
         /// <exception cref="System.IO.IOException"/>
         /// <exception cref="System.Exception"/>
         [NUnit.Framework.Test]

itext.tests/itext.forms.tests/itext/forms/PdfFormFieldTest.cs

@@ -565,5 +565,27 @@ public virtual void MultilineFormFieldNewLineTest() {
                 NUnit.Framework.Assert.Fail(errorMessage);
             }
         }
+
+        /// <exception cref="System.IO.IOException"/>
+        /// <exception cref="System.Exception"/>
+        [NUnit.Framework.Test]
+        public virtual void FillFormWithSameEmptyObjsForAppearance() {
+            String outPdf = destinationFolder + "fillFormWithSameEmptyObjsForAppearance.pdf";
+            String cmpPdf = sourceFolder + "cmp_fillFormWithSameEmptyObjsForAppearance.pdf";
+            PdfWriter writer = new PdfWriter(outPdf);
+            PdfReader reader = new PdfReader(sourceFolder + "fillFormWithSameEmptyObjsForAppearance.pdf");
+            PdfDocument pdfDoc = new PdfDocument(reader, writer);
+            PdfAcroForm acroForm = PdfAcroForm.GetAcroForm(pdfDoc, false);
+            acroForm.GetField("text_1").SetValue("Text 1!");
+            acroForm.GetField("text_2").SetValue("Text 2!");
+            acroForm.GetField("text.3").SetValue("Text 3!");
+            acroForm.GetField("text.4").SetValue("Text 4!");
+            pdfDoc.Close();
+            CompareTool compareTool = new CompareTool();
+            String errorMessage = compareTool.CompareByContent(outPdf, cmpPdf, destinationFolder, "diff_");
+            if (errorMessage != null) {
+                NUnit.Framework.Assert.Fail(errorMessage);
+            }
+        }
     }
 }