remove theashold

alpha theshold doesn't make sense for palette indexed colors.

src/ImageSharp/Formats/Png/IPngEncoderOptions.cs

@@ -40,11 +40,6 @@ namespace ImageSharp.Formats
         /// </summary>
         IQuantizer Quantizer { get; }
 
-        /// <summary>
-        /// Gets the transparency threshold.
-        /// </summary>
-        byte Threshold { get; }
-
         /// <summary>
         /// Gets a value indicating whether this instance should write
         /// gamma information to the stream.

src/ImageSharp/Formats/Png/PngEncoderOptions.cs

@@ -57,11 +57,6 @@ namespace ImageSharp.Formats
         /// </summary>
         public IQuantizer Quantizer { get; set; }
 
-        /// <summary>
-        /// Gets or sets the transparency threshold.
-        /// </summary>
-        public byte Threshold { get; set; } = 0;
-
         /// <summary>
         /// Gets or sets a value indicating whether this instance should write
         /// gamma information to the stream. The default value is false.

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

@@ -58,6 +58,46 @@ namespace ImageSharp.Tests.Formats.Png
             }
         }
 
+        [Theory]
+        [WithTestPatternImages(100, 100, PixelTypes.Color)]
+        public void CanSaveIndexedPngTwice<TColor>(TestImageProvider<TColor> provider)
+            where TColor : struct, IPixel<TColor>
+        {
+            // does saving a file then repoening mean both files are identical???
+            using (Image<TColor> source = provider.GetImage())
+            using (MemoryStream ms = new MemoryStream())
+            {
+                // image.Save(provider.Utility.GetTestOutputFileName("bmp"));
+                source.MetaData.Quality = 256;
+                source.Save(ms, new PngEncoder());
+                ms.Position = 0;
+                using (Image img1 = Image.Load(ms, new PngDecoder()))
+                {
+                    using (MemoryStream ms2 = new MemoryStream())
+                    {
+                        img1.Save(ms2, new PngEncoder());
+                        ms2.Position = 0;
+                        using (Image img2 = Image.Load(ms2, new PngDecoder()))
+                        {
+                            using (PixelAccessor<Color> pixels1 = img1.Lock())
+                            using (PixelAccessor<Color> pixels2 = img2.Lock())
+                            {
+                                for (int y = 0; y < img1.Height; y++)
+                                {
+                                    for (int x = 0; x < img1.Width; x++)
+                                    {
+                                        Assert.Equal(pixels1[x, y], pixels2[x, y]);
+                                    }
+                                }
+                            }
+                            // img2.Save(provider.Utility.GetTestOutputFileName("bmp", "_loaded"), new BmpEncoder());
+                            ImageComparer.CheckSimilarity(source, img2);
+                        }
+                    }
+                }
+            }
+        }
+
         [Theory]
         [WithTestPatternImages(300, 300, PixelTypes.All)]
         public void Resize<TColor>(TestImageProvider<TColor> provider)