Better tRNS coverage

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

@@ -77,6 +77,8 @@ namespace SixLabors.ImageSharp.Tests
 
             TestImages.Png.Bad.ChunkLength2,
             TestImages.Png.VimImage2,
+
+            TestImages.Png.Rgb24BppTrans,
         };
 
         public static readonly string[] TestImages48Bpp =
@@ -88,6 +90,7 @@ namespace SixLabors.ImageSharp.Tests
         public static readonly string[] TestImages64Bpp =
 {
             TestImages.Png.Rgba64Bpp,
+            TestImages.Png.Rgb48BppTrans
         };
 
         public static readonly string[] TestImagesGray16Bit =

tests/ImageSharp.Tests/TestImages.cs

@@ -30,13 +30,15 @@ namespace SixLabors.ImageSharp.Tests
             public const string Gray16Bit = "Png/gray-16.png";
             public const string GrayAlpha16Bit = "Png/gray-alpha-16.png";
             public const string GrayTrns16Bit = "Png/gray-16-tRNS.png";
+            public const string Rgb24BppTrans = "Png/rgb-8-tRNS.png";
             public const string Rgb48Bpp = "Png/rgb-48bpp.png";
+            public const string Rgb48BppInterlaced = "Png/rgb-48bpp-interlaced.png";
+            public const string Rgb48BppTrans = "Png/rgb-16-tRNS.png";
             public const string Rgba64Bpp = "Png/rgb-16-alpha.png";
             public const string CalliphoraPartial = "Png/CalliphoraPartial.png";
             public const string CalliphoraPartialGrayscale = "Png/CalliphoraPartialGrayscale.png";
             public const string Bike = "Png/Bike.png";
             public const string BikeGrayscale = "Png/BikeGrayscale.png";
-            public const string Rgb48BppInterlaced = "Png/rgb-48bpp-interlaced.png";
             public const string SnakeGame = "Png/SnakeGame.png";
             public const string Icon = "Png/icon.png";
             public const string Kaboom = "Png/kaboom.png";