Less memory with sub filter encoding

src/ImageSharp/Formats/Png/Filters/SubFilter.cs

@@ -44,11 +44,9 @@ namespace ImageSharp.Formats
         /// <param name="bytesPerPixel">The bytes per pixel.</param>
         /// <param name="bytesPerScanline">The number of bytes per scanline</param>
         /// <returns>The <see cref="T:byte[]"/></returns>
-        public static byte[] Encode(byte[] scanline, int bytesPerPixel, int bytesPerScanline)
+        public static byte[] Encode(byte[] scanline, byte[] result, int bytesPerPixel, int bytesPerScanline)
         {
             // Sub(x) = Raw(x) - Raw(x-bpp)
-            byte[] result = new byte[bytesPerScanline + 1];
-
             fixed (byte* scan = scanline)
             fixed (byte* res = result)
             {

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -355,7 +355,7 @@ namespace ImageSharp.Formats
 
             Tuple<byte[], int>[] candidates = new Tuple<byte[], int>[4];
 
-            byte[] sub = SubFilter.Encode(rawScanline, this.bytesPerPixel, bytesPerScanline);
+            byte[] sub = SubFilter.Encode(rawScanline, result, this.bytesPerPixel, bytesPerScanline);
             candidates[0] = new Tuple<byte[], int>(sub, this.CalculateTotalVariation(sub));
 
             byte[] up = UpFilter.Encode(rawScanline, previousScanline, result, bytesPerScanline);