Switch default quantizer for the bitmap encoder to Wu-quantizer

src/ImageSharp/Formats/Bmp/BmpEncoder.cs

@@ -30,7 +30,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
 
         /// <summary>
         /// Gets or sets the quantizer for reducing the color count for 8-Bit images.
-        /// Defaults to OctreeQuantizer.
+        /// Defaults to Wu Quantizer.
         /// </summary>
         public IQuantizer Quantizer { get; set; }
 

src/ImageSharp/Formats/Bmp/BmpEncoderCore.cs

@@ -98,7 +98,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
             this.memoryAllocator = memoryAllocator;
             this.bitsPerPixel = options.BitsPerPixel;
             this.writeV4Header = options.SupportTransparency;
-            this.quantizer = options.Quantizer ?? KnownQuantizers.Octree;
+            this.quantizer = options.Quantizer ?? KnownQuantizers.Wu;
         }
 
         /// <summary>

src/ImageSharp/Formats/Bmp/IBmpEncoderOptions.cs

@@ -25,7 +25,6 @@ namespace SixLabors.ImageSharp.Formats.Bmp
 
         /// <summary>
         /// Gets the quantizer for reducing the color count for 8-Bit, 4-Bit, and 1-Bit images.
-        /// Defaults to the Octree Quantizer.
         /// </summary>
         IQuantizer Quantizer { get; }
     }

tests/ImageSharp.Tests/Formats/Bmp/BmpEncoderTests.cs

@@ -212,11 +212,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Bmp
             // The Magick Reference Decoder can not decode 1-Bit bitmaps, so only execute this on windows.
             if (TestEnvironment.IsWindows)
             {
-                TestBmpEncoderCore(
+                TestBmpEncoderCore(provider, bitsPerPixel, supportTransparency: false);
-                    provider,
-                    bitsPerPixel,
-                    supportTransparency: false,
-                    quantizer: KnownQuantizers.Wu);  // using the wu quantizer, because the octree seems to have problems with just two colors.
             }
         }
 
@@ -230,11 +226,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Bmp
             // The Magick Reference Decoder can not decode 1-Bit bitmaps, so only execute this on windows.
             if (TestEnvironment.IsWindows)
             {
-                TestBmpEncoderCore(
+                TestBmpEncoderCore(provider, bitsPerPixel, supportTransparency: true);
-                    provider,
-                    bitsPerPixel,
-                    supportTransparency: true,
-                    quantizer: KnownQuantizers.Wu); // using the wu quantizer, because the octree seems to have problems with just two colors.
             }
         }
 
@@ -351,7 +343,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Bmp
                 {
                     BitsPerPixel = bitsPerPixel,
                     SupportTransparency = supportTransparency,
-                    Quantizer = quantizer ?? KnownQuantizers.Octree
+                    Quantizer = quantizer ?? KnownQuantizers.Wu
                 };
 
                 // Does DebugSave & load reference CompareToReferenceInput():