Added Skip extension to support streams that cannot seek.

src/ImageSharp/Common/Extensions/StreamExtensions.cs

@@ -0,0 +1,25 @@
+// <copyright file="StreamExtensions.cs" company="James Jackson-South">
+// Copyright (c) James Jackson-South and contributors.
+// Licensed under the Apache License, Version 2.0.
+// </copyright>
+
+namespace ImageSharp
+{
+    using System.IO;
+
+    internal static class StreamExtensions
+    {
+        public static void Skip(this Stream stream, int count)
+        {
+            if (stream.CanSeek)
+            {
+                stream.Position += count;
+            }
+            else
+            {
+                byte[] foo = new byte[count];
+                stream.Read(foo, 0, count);
+            }
+        }
+    }
+}

src/ImageSharp/Formats/Gif/GifDecoderCore.cs

@@ -59,7 +59,7 @@ namespace ImageSharp.Formats
             this.currentStream = stream;
 
             // Skip the identifier
-            this.currentStream.Seek(6, SeekOrigin.Current);
+            this.currentStream.Skip(6);
             this.ReadLogicalScreenDescriptor();
 
             if (this.logicalScreenDescriptor.GlobalColorTableFlag)
@@ -192,13 +192,13 @@ namespace ImageSharp.Formats
         /// <param name="length">The number of bytes to skip.</param>
         private void Skip(int length)
         {
-            this.currentStream.Seek(length, SeekOrigin.Current);
+            this.currentStream.Skip(length);
 
             int flag;
 
             while ((flag = this.currentStream.ReadByte()) != 0)
             {
-                this.currentStream.Seek(flag, SeekOrigin.Current);
+                this.currentStream.Skip(flag);
             }
         }
 

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -116,7 +116,7 @@ namespace ImageSharp.Formats
         {
             Image<TColor, TPacked> currentImage = image;
             this.currentStream = stream;
-            this.currentStream.Seek(8, SeekOrigin.Current);
+            this.currentStream.Skip(8);
 
             bool isEndChunkReached = false;