Ensure colormap cache is cleared

src/ImageSharp/Quantizers/OctreeQuantizer{TPixel}.cs

@@ -61,6 +61,7 @@ namespace SixLabors.ImageSharp.Quantizers
             this.colors = (byte)maxColors.Clamp(1, 255);
             this.octree = new Octree(this.GetBitsNeededForColorDepth(this.colors));
             this.palette = null;
+            this.colorMap.Clear();
 
             return base.Quantize(image, this.colors);
         }

src/ImageSharp/Quantizers/PaletteQuantizer{TPixel}.cs

@@ -58,6 +58,7 @@ namespace SixLabors.ImageSharp.Quantizers
         public override QuantizedImage<TPixel> Quantize(ImageFrame<TPixel> image, int maxColors)
         {
             Array.Resize(ref this.colors, maxColors.Clamp(1, 255));
+            this.colorMap.Clear();
             return base.Quantize(image, maxColors);
         }
 

src/ImageSharp/Quantizers/WuQuantizer{TPixel}.cs

@@ -139,6 +139,7 @@ namespace SixLabors.ImageSharp.Quantizers
 
             this.colors = maxColors.Clamp(1, 255);
             this.palette = null;
+            this.colorMap.Clear();
 
             try
             {