tdf#113448 fix PDF forms export

This fixes verapdf A-2 validation error '6.3.3-2 annotation
appearance dict is wrong'.

Controls that use type /FT/Btn must have sub-dicts as /N content,
instead of a direct stream reference.

Change-Id: If985644fe3d583e98a0b3a703e4b33dbf652f165
Reviewed-on: https://gerrit.libreoffice.org/69295
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>

vcl/source/gdi/pdfwriter_impl.cxx

@@ -4489,6 +4489,19 @@ bool PDFWriterImpl::emitAppearances( PDFWidget& rWidget, OStringBuffer& rAnnotDi
             rAnnotDict.append( "/" );
             rAnnotDict.append( dict_item.first );
             bool bUseSubDict = (dict_item.second.size() > 1);
+
+            // PDF/A requires sub-dicts for /FT/Btn objects (clause
+            // 6.3.3)
+            if( m_bIsPDF_A1 || m_bIsPDF_A2 )
+            {
+                if( rWidget.m_eType == PDFWriter::RadioButton ||
+                    rWidget.m_eType == PDFWriter::CheckBox ||
+                    rWidget.m_eType == PDFWriter::PushButton )
+                {
+                    bUseSubDict = true;
+                }
+            }
+
             rAnnotDict.append( bUseSubDict ? "<<" : " " );
 
             for (auto const& stream_item : dict_item.second)