Reduce memory usage in pixel map

Author: JimBobSquarePants

src/ImageSharp/Processing/Processors/Quantization/EuclideanPixelMap{TPixel}.cs

@@ -184,14 +184,14 @@ private static float DistanceSquared(Rgba32 a, Rgba32 b)
     /// The granularity of the cache has been determined based upon the current
     /// suite of test images and provides the lowest possible memory usage while
     /// providing enough match accuracy.
-    /// Entry count is currently limited to 4601025 entries (8MB).
+    /// Entry count is currently limited to 2335905 entries (4MB).
     /// </para>
     /// </remarks>
     private unsafe struct ColorDistanceCache : IDisposable
     {
         private const int IndexRBits = 5;
         private const int IndexGBits = 5;
-        private const int IndexBBits = 6;
+        private const int IndexBBits = 5;
         private const int IndexABits = 6;
         private const int IndexRCount = (1 << IndexRBits) + 1;
         private const int IndexGCount = (1 << IndexGBits) + 1;

tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs

@@ -499,7 +499,7 @@ public void Encode_AnimatedFormatTransform_FromGif<TPixel>(TestImageProvider<TPi
         // TODO: Find a better way to compare.
         // The image has been visually checked but the quantization pattern used in the png encoder
         // means we cannot use an exact comparison nor replicate using the quantizing processor.
-        ImageComparer.TolerantPercentage(0.12f).VerifySimilarity(output, image);
+        ImageComparer.TolerantPercentage(0.46f).VerifySimilarity(output, image);
 
         GifMetadata gif = image.Metadata.GetGifMetadata();
         PngMetadata png = output.Metadata.GetPngMetadata();

tests/Images/External/ReferenceOutput/BmpEncoderTests/Encode_8BitColor_WithOctreeQuantizer_rgb32.bmp

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:23a9d9233314ec08bd3e464f245e69d96566cbb12d2dba36c69bba483d6ba6b8
+oid sha256:11375b15df083d98335f4a4baf0717e7fdd6b21ab2132a6815cadc787ac17e7d
 size 9270

tests/Images/External/ReferenceOutput/BmpEncoderTests/Encode_8BitColor_WithWuQuantizer_rgb32.bmp

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:3052831cb18fecc26f54e29dfe19b538d2e0d3c104ddd7ec5bc8e0adcf56693c
+oid sha256:e063e97cd8a000de6830adcc3961a7dc41785d40cd4d83af10ca38d96e071362
 size 9270