Use more accuracy when calculating variance. Fix SixLabors#866 (SixLabors#874)

* Use more accuracy when calculating variance. Fix SixLabors#866

* Add unit tests

* Add test that fails with old image.

* Make IFrameQuantizer IDisposable

* Update GifEncoderCore.cs

Author: JimBobSquarePants

tests/ImageSharp.Tests/TestImages.cs

@@ -53,6 +53,7 @@ public static class Png
             public const string Gray2BitTrans = "Png/gray-2-tRNS.png";
             public const string Gray4BitTrans = "Png/gray-4-tRNS.png";
             public const string Gray8BitTrans = "Png/gray-8-tRNS.png";
+            public const string LowColorVariance = "Png/low-variance.png";
 
             // Filtered test images from http://www.schaik.com/pngsuite/pngsuite_fil_png.html
             public const string Filter0 = "Png/filter0.png";