#127 pdf/a-1a compliance

adds struct tree and markinfo dictionaries to support pdf/a-1a compliance.

src/UglyToad.PdfPig.Tests/Writer/PdfDocumentBuilderTests.cs

@@ -522,6 +522,34 @@
             }
         }
 
+        [Fact]
+        public void CanGeneratePdfA1AFile()
+        {
+            var builder = new PdfDocumentBuilder
+            {
+                ArchiveStandard = PdfAStandard.A1A
+            };
+
+            var page = builder.AddPage(PageSize.A4);
+
+            var font = builder.AddTrueTypeFont(TrueTypeTestHelper.GetFileBytes("Roboto-Regular.ttf"));
+
+            page.AddText("Howdy PDF/A-1A!", 10, new PdfPoint(25, 700), font);
+
+            var bytes = builder.Build();
+
+            WriteFile(nameof(CanGeneratePdfA1AFile), bytes);
+
+            using (var pdf = PdfDocument.Open(bytes, ParsingOptions.LenientParsingOff))
+            {
+                Assert.Equal(1, pdf.NumberOfPages);
+
+                Assert.True(pdf.TryGetXmpMetadata(out var xmp));
+
+                Assert.NotNull(xmp.GetXDocument());
+            }
+        }
+
         private static void WriteFile(string name, byte[] bytes)
         {
             try