Introduce a few more constants

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

@@ -51,7 +51,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
         [MethodImpl(InliningOptions.ShortMethod)]
         public void CheckBits()
         {
-            if (this.remainingBits < 16)
+            if (this.remainingBits < JpegConstants.Huffman.MinBits)
             {
                 this.FillBuffer();
             }
@@ -85,8 +85,8 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
         {
             // Attempt to load at least the minimum number of required bits into the buffer.
             // We fail to do so only if we hit a marker or reach the end of the input stream.
-            this.remainingBits += JpegConstants.Huffman.MinBits;
-            this.data = (this.data << JpegConstants.Huffman.MinBits) | this.GetBytes();
+            this.remainingBits += JpegConstants.Huffman.FetchBits;
+            this.data = (this.data << JpegConstants.Huffman.FetchBits) | this.GetBytes();
         }
 
         [MethodImpl(InliningOptions.ShortMethod)]

src/ImageSharp/Formats/Jpeg/JpegConstants.cs

@@ -251,9 +251,14 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             public const int RegisterSize = 64;
 
             /// <summary>
-            /// The minimum number of bits required by the <see cref="HuffmanScanBuffer"/>.
+            /// The number of bits to fetch when filling the <see cref="HuffmanScanBuffer"/> buffer.
             /// </summary>
-            public const int MinBits = 48;
+            public const int FetchBits = 48;
+
+            /// <summary>
+            /// The minimum number of bits allowed before by the <see cref="HuffmanScanBuffer"/> before fetching.
+            /// </summary>
+            public const int MinBits = RegisterSize - FetchBits;
 
             /// <summary>
             /// If the next Huffman code is no more than this number of bits, we can obtain its length