writes the eXIf chunk to the stream during encoding (#611)

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -221,6 +221,7 @@ namespace SixLabors.ImageSharp.Formats.Png
 
             this.WritePhysicalChunk(stream, image);
             this.WriteGammaChunk(stream);
+            this.WriteExifChunk(stream, image);
             this.WriteDataChunks(image.Frames.RootFrame, stream);
             this.WriteEndChunk(stream);
             stream.Flush();
@@ -523,6 +524,21 @@ namespace SixLabors.ImageSharp.Formats.Png
             }
         }
 
+        /// <summary>
+        /// Writes the eXIf chunk to the stream, if any EXIF Profile values are present in the meta data.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="stream">The <see cref="Stream"/> containing image data.</param>
+        /// <param name="image">The image.</param>
+        private void WriteExifChunk<TPixel>(Stream stream, Image<TPixel> image)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            if (image.MetaData.ExifProfile.Values.Count > 0)
+            {
+                this.WriteChunk(stream, PngChunkType.Exif, image.MetaData.ExifProfile.RawData);
+            }
+        }
+
         /// <summary>
         /// Writes the gamma information to the stream.
         /// </summary>

src/ImageSharp/MetaData/Profiles/Exif/ExifProfile.cs

@@ -3,7 +3,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Collections.ObjectModel;
 using System.IO;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Primitives;
@@ -95,6 +94,11 @@ namespace SixLabors.ImageSharp.MetaData.Profiles.Exif
         /// </summary>
         public ExifParts Parts { get; set; }
 
+        /// <summary>
+        /// Gets the byte data array containing the exif data.
+        /// </summary>
+        public byte[] RawData => this.data;
+
         /// <summary>
         /// Gets the tags that where found but contained an invalid value.
         /// </summary>