Pack the bits directly in GifImageDescriptor

This avoids the array allocation in PackedField

src/ImageSharp/Formats/Gif/GifEncoderCore.cs

@@ -290,7 +290,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
                 localColorTableFlag: true,
                 interfaceFlag: false,
                 sortFlag: false,
-                localColorTableSize: this.bitDepth); // Note: we subtract 1 from the colorTableSize writing
+                localColorTableSize: (byte)this.bitDepth); // Note: we subtract 1 from the colorTableSize writing
 
             var descriptor = new GifImageDescriptor(
                 left: 0,

src/ImageSharp/Formats/Gif/Sections/GifImageDescriptor.cs

@@ -81,16 +81,36 @@ namespace SixLabors.ImageSharp.Formats.Gif
             return MemoryMarshal.Cast<byte, GifImageDescriptor>(buffer)[0];
         }
 
-        public static byte GetPackedValue(bool localColorTableFlag, bool interfaceFlag, bool sortFlag, int localColorTableSize)
+        public static byte GetPackedValue(bool localColorTableFlag, bool interfaceFlag, bool sortFlag, byte localColorTableSize)
         {
-            var field = default(PackedField);
-
-            field.SetBit(0, localColorTableFlag);          // 0: Local color table flag = 1 (LCT used)
-            field.SetBit(1, interfaceFlag);                // 1: Interlace flag 0
-            field.SetBit(2, sortFlag);                     // 2: Sort flag 0
-            field.SetBits(5, 3, localColorTableSize - 1);  // 3-4: Reserved, 5-7 : LCT size. 2^(N+1)
-
-            return field.Byte;
+            /*
+            Local Color Table Flag     | 1 Bit
+            Interlace Flag             | 1 Bit
+            Sort Flag                  | 1 Bit
+            Reserved                   | 2 Bits
+            Size of Local Color Table  | 3 Bits
+            */
+
+            byte value = 0;
+
+            if (localColorTableFlag)
+            {
+                value |= 1 << 7;
+            }
+
+            if (interfaceFlag)
+            {
+                value |= 1 << 6;
+            }
+
+            if (sortFlag)
+            {
+                value |= 1 << 5;
+            }
+
+            value |= (byte)(localColorTableSize - 1);
+
+            return value;
         }
     }
 }

tests/ImageSharp.Tests/Formats/Gif/Sections/GifImageDescriptorTests.cs

@@ -0,0 +1,25 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using SixLabors.ImageSharp.Formats.Gif;
+
+using Xunit;
+
+namespace SixLabors.ImageSharp.Tests.Formats.Gif
+{
+    public class GifImageDescriptorTests
+    {
+
+        [Fact]
+        public void TestPackedValue()
+        {
+            Assert.Equal(128, GifImageDescriptor.GetPackedValue(true, false, false, 1));  // localColorTable
+            Assert.Equal(64,  GifImageDescriptor.GetPackedValue(false, true, false, 1));  // interfaceFlag
+            Assert.Equal(32,  GifImageDescriptor.GetPackedValue(false, false, true, 1));  // sortFlag
+            Assert.Equal(224, GifImageDescriptor.GetPackedValue(true,  true,  true, 1));  // all
+            Assert.Equal(7,   GifImageDescriptor.GetPackedValue(false, false, false, 8));
+            Assert.Equal(227, GifImageDescriptor.GetPackedValue(true, true, true, 4));
+            Assert.Equal(231, GifImageDescriptor.GetPackedValue(true, true, true, 8));
+        }
+    }
+}