Stackalloc bmp header

src/ImageSharp/Formats/Bmp/BmpEncoderCore.cs

@@ -66,16 +66,25 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                 reserved: 0,
                 fileSize: 54 + infoHeader.ImageSize);
 
-            byte[] buffer = new byte[40]; // TODO: stackalloc
-
+#if NETCOREAPP2_1
+            Span<byte> buffer = stackalloc byte[40];
+#else
+            byte[] buffer = new byte[40];
+#endif
             fileHeader.WriteTo(buffer);
 
+#if NETCOREAPP2_1
+            stream.Write(buffer.Slice(0, BmpFileHeader.Size));
+#else
             stream.Write(buffer, 0, BmpFileHeader.Size);
-
+#endif
             infoHeader.WriteTo(buffer);
 
+#if NETCOREAPP2_1
+            stream.Write(buffer.Slice(0, 40));
+#else
             stream.Write(buffer, 0, 40);
-
+#endif
             this.WriteImage(stream, image.Frames.RootFrame);
 
             stream.Flush();