add initial png support

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

@@ -529,6 +529,44 @@
             }
         }
 
+        [Fact]
+        public void CanWriteSinglePageWithPng()
+        {
+            var builder = new PdfDocumentBuilder();
+            var page = builder.AddPage(PageSize.A4);
+
+            var font = builder.AddStandard14Font(Standard14Font.Helvetica);
+
+            page.AddText("Piggy", 12, new PdfPoint(25, page.PageSize.Height - 52), font);
+
+            var img = IntegrationHelpers.GetDocumentPath("pdfpig.png", false);
+
+            var expectedBounds = new PdfRectangle(25, page.PageSize.Height - 300, 200, page.PageSize.Height - 200);
+
+            var imageBytes = File.ReadAllBytes(img);
+
+            page.AddPng(imageBytes, expectedBounds);
+
+            var bytes = builder.Build();
+            WriteFile(nameof(CanWriteSinglePageWithPng), bytes);
+
+            using (var document = PdfDocument.Open(bytes))
+            {
+                var page1 = document.GetPage(1);
+
+                Assert.Equal("Piggy", page1.Text);
+
+                var image = Assert.Single(page1.GetImages());
+
+                Assert.NotNull(image);
+
+                Assert.Equal(expectedBounds.BottomLeft, image.Bounds.BottomLeft);
+                Assert.Equal(expectedBounds.TopRight, image.Bounds.TopRight);
+
+                Assert.Equal(imageBytes, image.RawBytes);
+            }
+        }
+
         private static void WriteFile(string name, byte[] bytes)
         {
             try