Use constant to specify the size of the buffer

It's only one value for the fixed buffer size and the creation of the span -- so less error prone once the value needs to be updated.

src/ImageSharp/Formats/Gif/GifDecoderCore.cs

@@ -765,8 +765,9 @@ internal sealed class GifDecoderCore : IImageDecoderInternals
 
     private unsafe struct ScratchBuffer
     {
-        private fixed byte scratch[16];
+        private const int Size = 16;
+        private fixed byte scratch[Size];
 
-        public Span<byte> Span => MemoryMarshal.CreateSpan(ref this.scratch[0], 16);
+        public Span<byte> Span => MemoryMarshal.CreateSpan(ref this.scratch[0], Size);
     }
 }

src/ImageSharp/Formats/Jpeg/Components/Decoder/ArithmeticScanDecoder.cs

@@ -53,6 +53,7 @@ internal class ArithmeticScanDecoder : IJpegScanDecoder
 
     private ArithmeticDecodingTable[] acDecodingTables;
 
+    // Don't make this a ReadOnlySpan<byte>, as the values need to get updated.
     private readonly byte[] fixedBin = { 113, 0, 0, 0 };
 
     private readonly CancellationToken cancellationToken;

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -1412,8 +1412,9 @@ internal sealed class PngEncoderCore : IImageEncoderInternals, IDisposable
 
     private unsafe struct ScratchBuffer
     {
-        private fixed byte scratch[16];
+        private const int Size = 16;
+        private fixed byte scratch[Size];
 
-        public Span<byte> Span => MemoryMarshal.CreateSpan(ref this.scratch[0], 16);
+        public Span<byte> Span => MemoryMarshal.CreateSpan(ref this.scratch[0], Size);
     }
 }

src/ImageSharp/Formats/Webp/BitWriter/BitWriterBase.cs

@@ -260,8 +260,9 @@ internal abstract class BitWriterBase
 
     private unsafe struct ScratchBuffer
     {
-        private fixed byte scratch[4];
+        private const int Size = 4;
+        private fixed byte scratch[Size];
 
-        public Span<byte> Span => MemoryMarshal.CreateSpan(ref this.scratch[0], 4);
+        public Span<byte> Span => MemoryMarshal.CreateSpan(ref this.scratch[0], Size);
     }
 }

src/ImageSharp/Formats/Webp/Lossless/Vp8LEncoder.cs

@@ -1834,8 +1834,9 @@ internal class Vp8LEncoder : IDisposable
     /// </summary>
     private unsafe struct ScratchBuffer
     {
-        private fixed int scratch[256];
+        private const int Size = 256;
+        private fixed int scratch[Size];
 
-        public Span<int> Span => MemoryMarshal.CreateSpan(ref this.scratch[0], 256);
+        public Span<int> Span => MemoryMarshal.CreateSpan(ref this.scratch[0], Size);
     }
 }