Optimize png format detection

src/ImageSharp/Formats/Png/PngConstants.cs

@@ -36,5 +36,10 @@ namespace SixLabors.ImageSharp.Formats.Png
              0x1A, // EOF
              0x0A // LF
         };
+
+        /// <summary>
+        /// The header bytes as a big endian coded ulong.
+        /// </summary>
+        public const ulong HeaderValue = 0x89504E470D0A1A0AUL;
     }
 }

src/ImageSharp/Formats/Png/PngImageFormatDetector.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
+using System.Buffers.Binary;
 
 namespace SixLabors.ImageSharp.Formats.Png
 {
@@ -26,16 +27,7 @@ namespace SixLabors.ImageSharp.Formats.Png
 
         private bool IsSupportedFileFormat(ReadOnlySpan<byte> header)
         {
-            // TODO: This should be in constants
-            return header.Length >= this.HeaderSize &&
-                   header[0] == 0x89 &&
-                   header[1] == 0x50 && // P
-                   header[2] == 0x4E && // N
-                   header[3] == 0x47 && // G
-                   header[4] == 0x0D && // CR
-                   header[5] == 0x0A && // LF
-                   header[6] == 0x1A && // EOF
-                   header[7] == 0x0A;   // LF
+            return header.Length >= this.HeaderSize && BinaryPrimitives.ReadUInt64BigEndian(header) == PngConstants.HeaderValue;
         }
     }
 }