Fix local color table encoding.

src/ImageSharp/Formats/Gif/GifEncoderCore.cs

@@ -71,6 +71,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
             this.textEncoding = options.TextEncoding ?? GifConstants.DefaultEncoding;
             this.quantizer = options.Quantizer;
             this.ignoreMetadata = options.IgnoreMetadata;
+            this.colorTableMode = options.ColorTableMode;
         }
 
         /// <summary>
@@ -246,7 +247,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
         private void WriteLogicalScreenDescriptor<TPixel>(Image<TPixel> image, int transparencyIndex, bool useGlobalTable, Stream stream)
             where TPixel : struct, IPixel<TPixel>
         {
-            byte packedValue = GifLogicalScreenDescriptor.GetPackedValue(useGlobalTable, this.bitDepth, false, this.bitDepth - 1);
+            byte packedValue = GifLogicalScreenDescriptor.GetPackedValue(useGlobalTable, this.bitDepth - 1, false, this.bitDepth - 1);
 
             // The Pixel Aspect Ratio is defined to be the quotient of the pixel's
             // width over its height.  The value range in this field allows

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

@@ -108,7 +108,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
                 value |= 1 << 5;
             }
 
-            value |= (byte)(localColorTableSize - 1);
+            value |= (byte)localColorTableSize;
 
             return value;
         }

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

@@ -12,13 +12,13 @@ namespace SixLabors.ImageSharp.Tests.Formats.Gif
         [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));
+            Assert.Equal(129, GifImageDescriptor.GetPackedValue(true, false, false, 1));  // localColorTable
+            Assert.Equal(65,  GifImageDescriptor.GetPackedValue(false, true, false, 1));  // interfaceFlag
+            Assert.Equal(33,  GifImageDescriptor.GetPackedValue(false, false, true, 1));  // sortFlag
+            Assert.Equal(225, GifImageDescriptor.GetPackedValue(true,  true,  true, 1));  // all
+            Assert.Equal(8,   GifImageDescriptor.GetPackedValue(false, false, false, 8));
+            Assert.Equal(228, GifImageDescriptor.GetPackedValue(true, true, true, 4));
+            Assert.Equal(232, GifImageDescriptor.GetPackedValue(true, true, true, 8));
         }
     }
 }