try to fine-tune tolerance in PngEncoderTests + better Rgba64.ToString()

src/ImageSharp/PixelFormats/Rgba64.cs

@@ -290,7 +290,7 @@ namespace SixLabors.ImageSharp.PixelFormats
         /// <inheritdoc />
         public override string ToString()
         {
-            return this.ToVector4().ToString();
+            return $"({this.R},{this.G},{this.B},{this.A})";
         }
 
         /// <inheritdoc />

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

@@ -21,7 +21,7 @@ namespace SixLabors.ImageSharp.Tests
     {
         // This is bull. Failing online for no good reason. 
         // The images are an exact match. Maybe the submodule isn't updating?
-        private const float ToleranceThresholdForPaletteEncoder = 0.2273F;
+        private const float ToleranceThresholdForPaletteEncoder = 1.0F / 100;
 
         /// <summary>
         /// All types except Palette