Avoid bounds checks in GetBit()

src/ImageSharp/Formats/Tiff/Compression/Decompressors/T4BitReader.cs

@@ -6,6 +6,7 @@ using System.Buffers;
 using System.Collections.Generic;
 using System.IO;
 using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 using SixLabors.ImageSharp.Formats.Tiff.Constants;
 using SixLabors.ImageSharp.Memory;
 
@@ -815,7 +816,8 @@ namespace SixLabors.ImageSharp.Formats.Tiff.Compression.Decompressors
 
             Span<byte> dataSpan = this.Data.GetSpan();
             int shift = 8 - this.BitsRead - 1;
-            uint bit = (uint)((dataSpan[(int)this.Position] & (1 << shift)) != 0 ? 1 : 0);
+            ref byte dataAtPosition = ref Unsafe.Add(ref MemoryMarshal.GetReference(dataSpan), (int)this.Position);
+            uint bit = (uint)((dataAtPosition & (1 << shift)) != 0 ? 1 : 0);
             this.BitsRead++;
 
             return bit;

src/ImageSharp/Formats/Tiff/Compression/Decompressors/T6TiffCompression.cs

@@ -15,10 +15,6 @@ namespace SixLabors.ImageSharp.Formats.Tiff.Compression.Decompressors
     {
         private readonly bool isWhiteZero;
 
-        private readonly byte whiteValue;
-
-        private readonly byte blackValue;
-
         private readonly int width;
 
         /// <summary>
@@ -40,8 +36,6 @@ namespace SixLabors.ImageSharp.Formats.Tiff.Compression.Decompressors
             this.FillOrder = fillOrder;
             this.width = width;
             this.isWhiteZero = photometricInterpretation == TiffPhotometricInterpretation.WhiteIsZero;
-            this.whiteValue = (byte)(this.isWhiteZero ? 0 : 1);
-            this.blackValue = (byte)(this.isWhiteZero ? 1 : 0);
         }
 
         /// <summary>