Handle Bmp encoder padding.

src/ImageSharp/Common/Helpers/SimdUtils.Shuffle.cs

@@ -225,9 +225,9 @@ namespace SixLabors.ImageSharp
                 "Output span must be divisable by 3!");
 
             DebugGuard.IsTrue(
-                source.Length == dest.Length * 4 / 3,
+                dest.Length >= source.Length * 3 / 4,
                 nameof(source),
-                "Output span must be 3/4 the length of the input span!");
+                "Output span must be at least 3/4 the length of the input span!");
         }
 
         public static class Shuffle

tests/ImageSharp.Tests/Common/SimdUtilsTests.cs

@@ -164,8 +164,6 @@ namespace SixLabors.ImageSharp.Tests.Common
         public static readonly TheoryData<int> ArraySizesDivisibleBy8 = new TheoryData<int> { 0, 8, 16, 1024 };
         public static readonly TheoryData<int> ArraySizesDivisibleBy4 = new TheoryData<int> { 0, 4, 8, 28, 1020 };
         public static readonly TheoryData<int> ArraySizesDivisibleBy3 = new TheoryData<int> { 0, 3, 9, 36, 957 };
-
-
         public static readonly TheoryData<int> ArraySizesDivisibleBy32 = new TheoryData<int> { 0, 32, 512 };
 
         public static readonly TheoryData<int> ArbitraryArraySizes =